Gift parasite is a person who adds their name to a gift tag in order to claim partial credit for giving the gift.

“礼物寄生虫”指的是把名字加在礼物标签上写的送礼人名单里，以便能沾到一点功劳。

Example: 

I'm totally broke so I had to be a gift parasite and sign on that present you're giving Grandma.

我彻底没钱了，所以我只能做个“礼物寄生虫”，在你送给奶奶的礼物上加个名字。