I am trying to remove a gateway server from our environment.

I've pointed all agents to the RMS.
I've verified that none of the agentless managed computers are using it.
I've deleted all network devices that were using it.
I've uninstalled SCOM from the gateway server.

Now, when I run the gateway removal tool I get the error "Removal of the server failed because it is remotely managing devices and/or computers. From the agent management view, you must first delete, or change the proxy agent of all network devices or agentless managed computers".

If I attempt to delete it from the console it says "The management server is part of the communication topology and cannot be deleted".

So it seems that it is still being used by something but I can't figure out what. The network devices were just deleted a few hours ago so I'm wondering if it's possible I need to allow more time. Otherwise I am stumped.
Any help is greatly appreciated.

Than

I'd wait for the nightly garbage collection to run and try again. You could also fire up the  SCOM powershell and do this:

$foo=get-managementserver | where {$_.Name -ieq "mygatewayserver.contoso.com"}

get-agent -Managementserver $foo

And see what pops out.

That's SCOM 07, if you're on SCOM2012, I think the command is now get-scomagent