YAGS is my generic open source RPG, and has been licensed under the GPL since 2000 or so. It has been used to play fantasy, modern horror and science fiction. It's sort of snoozing at the moment, but I do have more plans for it once I've finished my current campaign.
The main site is here:
The 'source' is on GitLab:
https://gitlab.com/samuelpenn/yags
The source files are XML, using a modified version of Docbook (which I called YAGSbook). Since it's GPL, it is designed so that the PDFs can be generated using a completely opensource tool chain.