I'm trying to buy some games using left over balance on a visa gift card but it keeps erroring saying Oops Something Went Wrong. Do visa gift cards work on gog? I know it works as I've already used it online on a different site.

Nope. Prepaid gift cards can't be used to process international payments, and GOG is based outside the United States, so you'd need to use a full-fledged credit card.

A list of GOG's accepted forms of payment can be found here.