There's been several GG Steam games. Whether you could say they are good enough to be on Steam is another matter entirely. What I have seen to date are pretty much walking sims. I can understand this, as AI is not great yet, but as games they would leave many people unimpressed in general. If Unreal and Unity don't fit the bill, I doubt GG will fare well, unless by fitting the bill, you mean easier to use, in which case yes GG is certainly easier than both engines.
GG is a fun tool for making games though and as much as I sometimes complain about it, I do use it quite a lot and normally find it fun while doing so. I just find the limitations annoying and that progress is slow. You will find odd little quirks now and again, for instance use the menu on a standalone and the game sounds will suddenly stop playing. Little things like that sound like no real issue, but I wouldn't want to release a game that did that unless it was free, if I could help it (many issues you may not notice till too late).
For the price it's worth a look. The media included is pretty much worth it alone, when you look at store prices on the other sites. Still a long way to go though for it to be considered ready to make steam games. Performance wise as well, not the best. You can have a fairly smooth GG game, but most aren't. Most I have tried, have run pretty badly (20 fps or less). I always figure the author must have a beast and not noticed, or doesn't care. It's always a pleasant surprise if you download and play someones game and it is smooth. It does happen occasionally
Edit - Oh, yes you can make indoor levels. Lighting is a little odd though and normally needs some adjusting to work.
If you haven't seen much GG stuff in action check out my youtube channel. There's plenty on there to show what it's capable of.
https://www.youtube.com/user/RetroGamebloke
SPECS: Q6600 CPU. Nvidia 660GTX. 8 Gig Memory. Win 7.