I ran into a situation where I could not get a gateway server to approve via the Microsoft.EnterpriseManagement.GatewayApprovalTool.exe. It was erroring out saying that “The gateway name already exists as a computer instance.”.
I am getting Event ID 20000 on the Management Server and Event IDs 20070 and 21016 on the gateway.
You will need to remove the computer name from the Agent Manged list. The supported method of doing this is to delete from the admin console(Administration > Device Manager > Agent Managed). But in some case you can’t find it in the console. If it is the case you need go the database for Ops Mgr, find the tabel called dbo.MT_Computer and open it. Find PrincipalName where it match and delete the row