As long as the game is a code you wrote or have and does not need advanced graphics just console output ("character" graphics) it should be fine.
Such a game could be word games, logic games, etc. Obviously this does not allow first person shooters, MMO games, etc.
Online GDB is a compiler (well, set of compilers) so it's here to compile a code for you and offer an editor for it.
Online GDB is not to run existing executables.