Sorry to be the bearer of bad news but…
Despite being recorded in my /s2member-logs there is no gift coupon use being logged via log_user_coupons_use($user_id, $coupon) with s2member_coupon_codes or s2member_coupon_uses in the users meta data.
All entries there seem to be an empty array
a:0:{}
And users I can see who have registered with a gift code do not seem to have had their coupon recorded in their profile either…