I have come up with a solution
1. changed the columns size and added and extra column in line 60 of rep107:
    $cols = array(4, 60, 195, 260, 285, 325, 390, 450, 515);
2. added the Gross amount calcuations ( hardcoded tax multiplier in file, but works for me for now) in rep107 as per below from line 114 onwards:
                $DisplayPrice = number_format2($myrow2["unit_price"],$dec);
                $DisplayQty = number_format2($sign*$myrow2["quantity"],get_qty_dec($myrow2['stock_id']));
                $DisplayNet = number_format2($Net,$dec);
             //Calculate Gross number
             $DisplayGross = number_format2($Net*1.16,$dec);                
                if ($myrow2["discount_percent"]==0)
                      $DisplayDiscount ="";
                else
                      $DisplayDiscount = number_format2($myrow2["discount_percent"]*100,user_percent_dec()) . "%";
                $rep->TextCol(0, 1,    $myrow2['stock_id'], -2);
                $oldrow = $rep->row;
                $rep->TextColLines(1, 2, $myrow2['StockDescription'], -2);
                $newrow = $rep->row;
                $rep->row = $oldrow;
                if ($Net != 0.0 || !is_service($myrow2['mb_flag']) || !isset($no_zero_lines_amount) || $no_zero_lines_amount == 0)
                {
                    $rep->TextCol(2, 3,    $DisplayQty, -2);
                    $rep->TextCol(3, 4,    $myrow2['units'], -2);
                    $rep->TextCol(4, 5,    $DisplayPrice, -2);
                    $rep->TextCol(5, 6,    $DisplayDiscount, -2);
                    $rep->TextCol(6, 7,    $DisplayNet, -2);
                            //Display Gross Number
                            $rep->TextCol(7, 8,    $DisplayGross, -2);    
3. Change the row descriptions in doctext.inc line 30
    // default item column headers
    $this->headers = array(_("Item Code"), _("Item Description"), _("Quantity"),
        _("Unit"), _("Price"), _("Discount %"), _("Net"), _("Gross"));