How to disable Game Center process (gamed) on OS X 10.11 El Capitan?
Disable the LaunchAgent (not LaunchDaemons) and reboot your machine (without sudo):

$ launchctl unload /System/Library/LaunchAgents/com.apple.gamed.plist


In case you want to enable it again at one point:

$ launchctl load /System/Library/LaunchAgents/com.apple.gamed.plist