UPBGE. Pretty old engine, but tutorials are abundant and it's 100% free and open source. If you have ANY prior coding experience (visual or script) it's a breeze to learn, and the only learning curve that I found was addons. The default game engine is good for starter games, but it's harder to get top-of-the-line results. With some community-created mods it's at least competitive with Unity (if not slightly better because who uses C#). It's no Unreal, but UPBGE is a great option for new devs who don't need all the features, and it's based on Blender so it's been around forever. Stability is something you might want to look into if you get an old version, so I'd recommend either 2.5b or the latest release.
Blender 2.79 is from 2017, and is still available for download. Might solve the whole stability thing.