Just to be clear.
You're trying to unscrew this thread cover right?
That thing isn't secured by anything else but thread. So if it doesn't unscrew, threads were probably damaged at the factory or during your adventures in removal. So really your only option left is probably to replace the part itself. It's too bad because that gun is a beast. Nothing comes close to it in performance in the 300$ price point. I know my local shop would take care of something like this under warranty, not sure about whoever you bought it from. You can contact G&G directly, I hear they have great customer service.