Apple - 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

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

The problem is that the accepted answer given above:

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

no longer works. I have also seen it using the -w flag. In any case, in recent updates, I think around El Capitan or Yosemite, you instead get a message

Operation not permitted while System Integrity Protection is engaged

System Integrity Protection is also known by the acronym SIP. To disable it you need to boot into the Recovery OS and run csrutil disable.

I have done this sequence in previous releases and it did lead to being able to disable gamed via the launchctl command.

I suppose that after gamed is disabled, one could re-enable SIP. Maybe that would be the best of both worlds, although it is a tedious process all in all.