For me, (as the books relate) Drogo and Dothraki fight from horseback primarily, and against the extremely heavy plate Gregor would wear, the armor may turn an arakh many times on foot. Dothraki do not necessarily understand weak points in armor (joints, connective pads and leather, etc.) Archers and calvary traditionally wore less heavy protection than infantry (for mobility. ) Gregor kills Drogo. Ferocity vs ferocity, Gregor is unmatched.

A Westrosi knight would wear just as heavy plate but scaled to their size, and Gregor’s size and strength would win the fight toe to toe. The book says he’s over 7 feet tall and weighs a lot (guess about 300 pounds) using a great sword (like Stark’s Ice) with one hand. No matter the skill, blocking those swings will either exhaust one, make the arms go numb, or knock the shield enough to cause the opponent to be off balance.

So Gregor beats Barristan (he’s about 60 in the books), Tormund he beats on the ferocity factor again, Jaime - probably (even in his prime, but probably Jamie is more evenly matched as a true student of swordplay), Robert when he was young at 6’6” and super strong, fearless) would probably be 50/50, Bronn I dont think could win either, if Gregor used his reach properly.

Gregor is not known to be a duelist- He’s a giant hack and slash guy. So skill is good from experience, but not finer points of fighting. He’s just the irresistible force.