Isn't the Build engine also FOSS now? Edit: it isn't
https://steamdb.info/tech/Engine/Build/
IdTech 1-4 are all also FOSS these days:
https://steamdb.info/tech/Engine/idTech1/
https://steamdb.info/tech/Engine/idTech2/
https://steamdb.info/tech/Engine/idTech2_5/