"The gateway name already exists as a computer instance."
Basically as the error says your operations manager server already knows about the server, normally as in my case this is because you have installed the SCOM agent already. Unfortunately just removing from SCOM will not actually work, as this doesn’t look to delete the server from the SQL database.
So login to your SQL server and find the operations manager database, find the table called “MT_computer table” and do a database select so you can find the offending row.
Once you have found the row I would suggest deleteing via the ID with something like:
DELETE FROM MT_Computer WHERE BaseManagedEntityID='[Replace with the ID to delete]'