Update:
Ok so I went ahead and started making some edits to the files you mentioned apmuthu. However, now the site is coming up as a blank page, so I think I may have broken something in the code of one of the *_db files. Here is a rundown of all the edits I've made.
sales_order_db.inc
Changed lines 82-90 to include gross and gds
        $sql = "INSERT INTO ".TB_PREF."sales_order_details (order_no, trans_type, stk_code, description, unit_price, quantity, gross, discount_percent, gds) VALUES (";
        $sql .= $order_no . ",".$order->trans_type .
                ",".db_escape($line->stock_id).", "
                .db_escape($line->item_description).", $line->price,
                $line->quantity,
                $line->gross,
                $line->discount_percent,
                $line->gds");
        db_query($sql, "order Details Cannot be Added");
Changed lines 234-247 to include gross and gds
        $sql = "INSERT INTO ".TB_PREF."sales_order_details
         (id, order_no, trans_type, stk_code, description, unit_price, quantity, gross,
          discount_percent, gds, qty_sent)
         VALUES (";
        $sql .= db_escape($line->id ? $line->id : 0) . ","
          .$order_no . ",".$order->trans_type.","
          .db_escape($line->stock_id) . ","
          .db_escape($line->item_description) . ", "
          .db_escape($line->price) . ", "
          .db_escape($line->quantity) . ", "
          .db_escape($line->gross) . ", "
          .db_escape($line->discount_percent) . ", "
          .db_escape($line->gds) . ", "
          .db_escape($line->qty_done) ." )";
        db_query($sql, "Old order Cannot be Inserted");
    } /* inserted line items into sales order details */
Changed lines 327-343 to include gross and gds
function get_sales_order_details($order_no, $trans_type) {
    $sql = "SELECT id, stk_code, unit_price, "
        .TB_PREF."sales_order_details.description,"
        .TB_PREF."sales_order_details.quantity, gross,
          discount_percent, gds,
          qty_sent as qty_done, "
        .TB_PREF."stock_master.units,"
        .TB_PREF."stock_master.mb_flag,"
        .TB_PREF."stock_master.material_cost + "
            .TB_PREF."stock_master.labour_cost + "
            .TB_PREF."stock_master.overhead_cost AS standard_cost
    FROM ".TB_PREF."sales_order_details, ".TB_PREF."stock_master
    WHERE ".TB_PREF."sales_order_details.stk_code = ".TB_PREF."stock_master.stock_id
    AND order_no =" . db_escape($order_no) 
        ." AND trans_type = " . db_escape($trans_type) . " ORDER BY id";
    return db_query($sql, "Retreive order Line Items");
}
Changed lines 383-390 to include gross and gds
    $result = get_sales_order_details($order_no, $order->trans_type);
    if (db_num_rows($result) > 0)
    {
        $line_no=0;
        while ($myrow = db_fetch($result))
        {
            $order->add_to_cart($line_no,$myrow["stk_code"],$myrow["quantity"],
                $myrow["unit_price"], $myrow["discount_percent"], $myrow["gds"], $myrow["gross"],
                $myrow["qty_done"], $myrow["standard_cost"], $myrow["description"], $myrow["id"] );
        $line_no++;
        }
    }
    return true;
}
sales/view/view_invoice.php
Changed lines 107-145
-Edited $th array to show Confirmation, Source, Avg Rate, Stay, Gross, Our Charge, GDS, and Total for the table headings.
-Added lines 141 to reflect math for Gross cell.
        amount_cell($myrow2["unit_price"] * $myrow2["quantity"]);
-Added line 143 for GDS.
        amount_cell($myrow2["gds"]);
sales/includes/ui/sales_order_ui.inc
Edited lines 144-145
    $th = array(_("Confirmation"), _("Source"), _("Avg Rate"), _("Stay"), _("Gross"), $order->tax_included ? _("Our Charge") : _("Price before Tax"), _("GDS"), _("Total"),);
Changed lines 163-218. Not sure if my additions are valid or not though.
        $line_total = round($stock_item->qty_dispatched * $stock_item->price * (1 - $stock_item->discount_percent),
           user_price_dec());
        $qoh_msg = '';
        if (!$editable_items || $id != $line_no)
        {
            if (!$SysPrefs->allow_negative_stock() && is_inventory_item($stock_item->stock_id) && 
                $order->trans_type != ST_SALESORDER && $order->trans_type!=ST_SALESQUOTE) {
                $qoh = get_qoh_on_date($stock_item->stock_id, 
                    $_POST['Location'], $_POST['delivery_date']);
                if (($stock_item->qty_dispatched -$stock_item->qty_done) > $qoh)
                {
                    // oops, we don't have enough of one of the component items
                    start_row("class='stockmankobg'");
                    $qoh_msg .= $stock_item->stock_id . " - " . $stock_item->item_description . ": " .
                    _("Quantity On Hand") . " = " 
                    . number_format2($qoh, get_qty_dec($stock_item->stock_id)) . '<br>';
                    $has_marked = true;
                 } else 
                    alt_table_row_color($k);
            } else {
                alt_table_row_color($k);
            }
            view_stock_status_cell($stock_item->stock_id);
            //label_cell($stock_item->item_description, "nowrap" );
            label_cell($stock_item->item_description );
            amount_cell($stock_item->price);
            $dec = get_qty_dec($stock_item->stock_id);
            qty_cell($stock_item->qty_dispatched, false, $dec);
            if ($order->trans_no!=0)
                qty_cell($stock_item->qty_done, false, $dec);
            //label_cell($stock_item->units);
                    amount_cell($stock_item->price * $stock_item->qty_dispatched);
            percent_cell($stock_item->discount_percent * 100);
                amount_cell($stock_item->gds);
            amount_cell($line_total);
            if ($editable_items)
            {
                edit_button_cell("Edit$line_no", _("Edit"),
                _('Edit document line'));
                delete_button_cell("Delete$line_no", _("Delete"),
                _('Remove line from document'));
            }
            end_row();
        }
        else
        {
            sales_order_item_controls($order, $k,  $line_no);
        }
        $total += $line_total;
    }
I know I need to edit/change lines 489-572 but I don't know what or how to do so (unsure of syntax, looking for any help!)
function sales_order_item_controls(&$order, &$rowcounter, $line_no=-1)
{
    global $Ajax;
    alt_table_row_color($rowcounter);
    $id = find_submit('Edit');
    if ($line_no!=-1 && $line_no == $id) // edit old line
    {
        $_POST['stock_id'] = $order->line_items[$id]->stock_id;
        $dec = get_qty_dec($_POST['stock_id']);
        $_POST['qty'] = number_format2($order->line_items[$id]->qty_dispatched, $dec);
        $_POST['price'] = price_format($order->line_items[$id]->price);
        $_POST['Disc'] = percent_format($order->line_items[$id]->discount_percent*100);
        $units = $order->line_items[$id]->units;
        $_POST['item_description'] = $order->line_items[$id]->item_description;
        hidden('stock_id', $_POST['stock_id']);
        label_cell($_POST['stock_id']);
        if ($order->line_items[$id]->descr_editable)
            text_cells(null,'item_description', null, 45, 150);
        else {
            hidden('item_description', $_POST['item_description']);
            label_cell($_POST['item_description']);
        }
//        } else {
//            sales_items_list_cells(null,'item_description', null, false, true);
//        }
        //label_cell($order->line_items[$line_no]->item_description, "nowrap");
        $Ajax->activate('items_table');
    }
    else    // prepare new line
    {
        sales_items_list_cells(null,'stock_id', null, false, true);
        if (list_updated('stock_id')) {
                $Ajax->activate('price');
                $Ajax->activate('units');
                $Ajax->activate('qty');
                $Ajax->activate('line_total');
        }
        $item_info = get_item_edit_info($_POST['stock_id']);
        $units = $item_info["units"];
        $dec = $item_info['decimals'];
        $_POST['qty'] = number_format2(1, $dec);
        $price = get_kit_price($_POST['stock_id'],
            $order->customer_currency, $order->sales_type,
            $order->price_factor, get_post('OrderDate'));
        $_POST['price'] = price_format($price);
        // default to the customer's discount %
        $_POST['Disc'] = percent_format($order->default_discount * 100);
    }
    qty_cells(null, 'qty', $_POST['qty'], null, null, $dec);
    if ($order->trans_no!=0) {
        qty_cell($line_no==-1 ? 0 :$order->line_items[$line_no]->qty_done, false, $dec);
    }
    label_cell($units, '', 'units');
    amount_cells(null, 'price');
    small_amount_cells(null, 'Disc', percent_format($_POST['Disc']), null, null, user_percent_dec());
    $line_total = input_num('qty') * input_num('price') * (1 - input_num('Disc') / 100);
    amount_cell($line_total, false, '','line_total');
    if ($id!=-1)
    {
        button_cell('UpdateItem', _("Update"),
                _('Confirm changes'), ICON_UPDATE);
        button_cell('CancelItemChanges', _("Cancel"),
                _('Cancel changes'), ICON_CANCEL);
        hidden('LineNo', $line_no);
        set_focus('qty');
    }
    else
    {
        submit_cells('AddItem', _("Add Item"), "colspan=2 align='center'",
            _('Add new item to document'), true);
    }
    end_row();
}
I have looked at sales/includes/db/sales_invoice_db.inc but I don't really know what or how to change the values there to include the two new values I want. Any thoughts, advice, tips, etc are greatly appreciated. 
As I said before, right now I'm getting a blank page whenever I try to access any pages on the site, I can't even log in. I think it's related to the sales_order_db.inc file that I edited, but I'm not sure. If it is, does anyone have any ideas why this might be? Thanks for all your help!
I wish you all a great holiday.