The statements you have given are not correct PHP syntax, but we would need to know your exact variable names to give a corrected version.
You would say something like
PHP Code:
if ($gift_certificate_amount > 0) {
$customized_product_total = $myprice - $gift_certificate_amount;
}
No "else" is required because if the amount is zero you don't need to do anything.
You could also simply say
PHP Code:
$customized_product_total = $myprice - $gift_certificate_amount;
and if the GC is zero, it will leave the total as is. Very simple.
Substitute your actual variable names if they are different, and double check spellings. You had several typos in your posted version - they would be fatal to your code.
Good luck.