When I worked at a gunsmithing shop I used a barrel clamp to remove Grenade launchers with. If you don't have that or have access to one then get a hold of a smooth jawed pipe wrench.(doesn't have any teeth)
Remove the pin,
Put the action in a vise.
Set the smooth jawed pipe wrench on the GL and put tension on it with one hand.
Then whack the end of the pipe wrench with a hammer to break it loose.
It should then unscrew without any problems and not leave any marks.
I have done this many times with no problems. Also, remember that the GL is RH thread so it will unscrew counter clockwise.