Currently I am developing a game in UPBGE. So can I publish my game without any cost? Is there any royalty in using UPBGE.
Welcome 
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you may just read in the docu: https://upbge.org/#/documentation/docs/latest/manual/manual/about/index.html under license …
I did not find my answer in that documentation.
Any software using some GPL or MIT license can be changes itself and used by everyone. The only software i’m aware of which claims some rights on the work you do is something like a game level editor. And mostly this is for relieve them of your claims.
If you make your own artwork and don’t copy any story or behaviour you are free to get all he problems a provider can face in this market… except from the maker of blender or UPBGE…
(this is no legal advice… of course not: i’m no lawyer and if i were you would have to pay me)
And yes maybe the UPBGE team should make a page like https://www.blender.org/about/license/ … hmm but since it is derived from Blender…