Our Development team is looking into a way to add a custom field to our shopping cart, into which the buyer could enter info before they proceed to checkout; however, I cannot speak for them regarding when or if this feature may become functional, and we are also limited to what data fields all our payment processors such as PayPal can receive from us and transmit back to us after payment is completed.
We have very little influence over the behavior of checkout itself on third-party payment sites such as PayPal. If your buyers could have a free gift card sent somewhere other than the shipping address they provide for the rest of their order, I might suggest you could add these lines to your View Cart code on every page, just before the "// -->" line in our standard View Cart code, reminding buyers to add Special Instructions during their PayPal checkout:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#tdPmnt").attr("innerHTML",
"Remember to add <b>Special Instructions</b> during checkout explaining where to send your free gift card!");
}