How to disable Game Center process (gamed) on OS X 10.11 El Capitan?
Solution 1:
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
Solution 2:
According to this thread you can do:
defaults write com.apple.gamed Disabled -bool true
And to check the current state of the daemon use:
defaults read com.apple.gamed Disabled