The Content Monitor shows Operational Status 0 and 1 , 0 means GUP is not serving as GUP to the clients.The Status 0- means GUP is offline or not communicating with SEPM.
These 2 Field are enough to determine GUP is having an issue and clients behind those GUPs won't update.
Additionally you can check which GUP is having disk space issue, when ws its last check in with SEPM and what is definitons on it.
basically Content Monitor is more less to check if GUP is working properly.