What are you making? Isn't GMS for helping to add support for Google things like Gmail and Chrome? Are you waiting for someone who is a pro with GMS to respond and give a comparison between that and C2? It's unclear what you want, all I can say is Construct 2 is powerful and has strong support for exporting to Apple and Google platforms and fits in well with third-party plugins to get your game or app up on the stores.
I'm assuming GMS is GameMaker: Studio
Hello guys,
I was working with GMS before, but hit headfirst a major snag. It charges money I can't spend for the module to publish to Macintosh.
...
If I may ask a question now, would it be worthwhile to master multiple systems, or should I just make a choice and stick to it?
-Tunicate
You'll need to purchase the C2 license in order to export to Mac. And its not always straight forward to get it to work, you might want to search the forums to see what issues people have experienced.
I don't see any harm in learning two different systems, though.