Old games

GPL source code:

Total Anihilation:

1) https://ru.wikipedia.org/wiki/Spring_Engine
2) https://springrts.com/wiki/Building_Spring_on_Linux

WarZone 2100:
1) http://developer.wz2100.net/wiki/CompileGuideLinux

Magic Carpet 2:
1) https://github.com/turican0/remc2

Magic carpet clone in Unity:

1) https://www.reddit.com/r/Unity3D/comments/jhgeup/come_along_on_a_magic_carpet_ride/

2) https://github.com/keyser84/UnityCarpet

3) https://assetstore.unity.com/packages/tools/game-toolkits/magic-carpet-3d-fps-game-kit-129894

4) https://stackoverflow.com/questions/40950188/magic-carpet-effect

Magic Carpet:

1) Magic Carpet https://www.old-games.ru/game/199.html

2) https://en.wikipedia.org/wiki/Bullfrog_Productions

3) https://en.wikipedia.org/wiki/Magic_Carpet_(video_game)

Doom etc.:

1) Игры id Software https://habr.com/ru/post/137442/

Descent:

1) https://store.steampowered.com/app/273570/Descent/

2) https://en.wikipedia.org/wiki/Descent_(video_game)

другие gpl-клоны и бывшие проприетарные:
1) https://en.wikipedia.org/wiki/List_of_game_engine_recreations
2) https://osgameclones.com/