Vader is the only he ever dueled and he duelled him twice..
Once in ESB when Vader toyed with him, he hit Vader and lost his arm. Then in ROTJ which is one of the most debated duels of all times and because I don't want to get into this discussion I will say that he hasn't beaten Vader when Vader was fighting his best.
And what form? What form did he learn? What form looks anything like what he did in ROTJ? And he doens't have speed in his form.
GG has fought Jedi Masters that had trained with a lightsaber for years and he killed them, he took them out sometimes many at the same time. Luke wouldn't be able to block the attack of four lightsabers, he wouldn't be able to move fast enough to block GG his attacks. There is nothing we have seen that indicates he has the speed or the skill to block anything GG would throw at him.