Starting point would be /beermoney on reddit, there are threads about changing gift cards etc because that sub is dedicated to lots of different online sites where you get paid and not all pay you directly but give you gift cards.
One of the first places they'll send you is to another sub made for the purpose of exchaning GC
https://www.reddit.com/r/giftcardexchange/
But it isn't a business like some others that buy/resell gift cards.
This is a question similar to yours from last year I don't know how reliable any of the suggestions are. I tend to avoid sites that hand out GC I don't want but haven't gotten into all the online opportunities.
https://www.reddit.com/r/beermoney/comments/aaym2k/alternatives_to_gift_card_exchange/