In your Column Grid version of /includes/modules/your_template/product_listing.php, find this section:
PHP Code:
// Used for Column Layout (Grid Layout) add on module
$column = 0;
if (PRODUCT_LISTING_LAYOUT_STYLE == 'columns') {
if ($num_products_count < PRODUCT_LISTING_COLUMNS_PER_ROW || PRODUCT_LISTING_COLUMNS_PER_ROW == 0 ) {
$col_width = floor(100/$num_products_count) - 0.5;
} else {
$col_width = floor(100/PRODUCT_LISTING_COLUMNS_PER_ROW) - 0.5;
}
}
and change both instances of
floor(100/...) - 0.5
to
floor(100/...) - 1.5
PHP Code:
// Used for Column Layout (Grid Layout) add on module
$column = 0;
if (PRODUCT_LISTING_LAYOUT_STYLE == 'columns') {
if ($num_products_count < PRODUCT_LISTING_COLUMNS_PER_ROW || PRODUCT_LISTING_COLUMNS_PER_ROW == 0 ) {
$col_width = floor(100/$num_products_count) - 1.5;
} else {
$col_width = floor(100/PRODUCT_LISTING_COLUMNS_PER_ROW) - 1.5;
}
}
Adjust the 1.5 as required to work with the margin you want.
There are several ways you can tweak this calculation; experiment if you wish once you understand the logic.