We can’t say, but my best bet is yes.
Thor is huge. He’s 6’9 and the worlds strongest man, as well as being skilled in Icelanding Wrestling.
The Book Mountain, however, is well over the 7 foot mark, and weighs, at a minimum, 420 pounds, probably more, given how wide across he is. High doesn’t mean too much, but given how he is this tall and still broad and well muscled means he is huge
We get very little lifting feats for Gregor, but Thor has recently beaten the world record for dead lift, over a thousand pounds. Gregor’s dead lift will be nothing in comparison.
There is a reason boxers, MMA Fighters and the like aren’t so freakishly huge as weightlifters. At a certain point, maintaining that muscle mass will simply be too great a waste of time. Gregor’s Muscle is all practical. Thor can squat, barbell, and deadlift more weight than the Mountain, but Gregor can out fight, out run, and outcompete Thor in any form of battle