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Looking at the license, I think you can sell your games made in this engine.

I don't see any problem with making long game.

In GB Studio you are restricted to 4 colors but you can change them

I don't known if you can make a JRPG, I don't really known limitations of this game engine

More information you should be able to find here:

sorry, I meant turn based RPG. I get my genre's mixed up, sorry.