I think GameCenter support is very poor on GameSparks. I've gotten the addon that will allow me to authenticate via GameCenter but any social requests (such as SocialLeaderboardData) seem to exclude GameCenter friends. This seems odd as it appears to be stored in external authentication.
With the current system, if you wanted to show friends leaderboards which included FB and GC, you need to make a social leaderboard request to get FB data, then get GameCenter friend id's, make a request to associate these GC friend id's to GameSparks accounts, and then make a leaderboard request with the GameSparks id's for the GameCenter friends. And after all that, you need to manually sort the results yourself (which almost defeats the convenience of leaderboards).
I think this system can be improved upon and make it more streamlined like with Facebook.
Currently the GameCenterConnectRequest doesn't handle users friends so that is why they don't show up in the SocialLeaderboardDataRequest. Users added to a social team will be considered game friends and will show up in Social Leaderboards. If you can manually identify the users you could add them to a users team for this purpose. We'll pass on your feedback to the dev team in the meantime.