Offline Gatekeepers have been detected
After upgrading, you may notice this message pop up:

This can happen for several reasons.
Stale gatekeeper that no longer exists
In this instance you will go to active directory with and enable these 2 options:

Then navigate here and delete the server name of the stale gatekeeper under the gatekeepers container:

The Specops Authentication gatekeeper service is not started

Port open between gatekeepers
Lastly check that TCP port 6003 is open between the gatekeeper servers.