Quickest way would be to shut server down, delete your server gmod/data/ulib/groups.txt , then restart server and let ULX(Ulib actually through ULX) rebuild your default groups file.
Then if needed, rebuild any other custom groups.
All. Gmod has no idea what 'owner' is. It's a group made up by some mods/admin systems.
Gmod, by default, only knows 'superadmin', 'admin' and (though not officially titled), 'user'.
Don't go deleting any of the default groups, as code within Gmod (even WITHOUT any other mod) looks for those three groups to exist.