I think the best thing to do is try it with the gamecenter options. I believe the plugin is designed to be more generic (leaderboards, achievements, purchases, etc) and then when you cloud compile it takes note of which options you select in the cloud compiler and compiles each distribution differently based on your selections. Then you just need to name your leaderboard the same on both Google Play and GameCenter and then you can just say, post this score to this leaderboard in C2. I think the name gamecenter in the plugin is confusing.
I'm not sure if I am explaining that very well. Does it make sense?
When I have time I plan on trying it myself.