There are a fair amount of open source games out there and some great lists but I mainly wanted to highlight with this comment that itch and steam both have ways to find them. Hell I was just playing one earlier to check out the new content (drl).
Steam has this handy curator: https://store.steampowered.com/curator/38475471-Libre-Open-S...
Itch has this filter: https://itch.io/games/tag-open-source
As a bonus here are some random non abandoned lists I found with quick search:
https://en.wikipedia.org/wiki/List_of_open-source_video_game...