If a Guyver boosts the host's power a hundredfold, in Sho's case he should be able to lift about 5 tons. Remember, Sho's a teenager and not a particularly big one. Tetsuro and Agito are both taller than he is. Even bio-boosted he's, what, 1.75 m tall? (checks) 1.74 m. Even if that's measured from the top of the helmet and not the horn, in human form Sho should be only about 1.65, tops. Reckon he could lift 50 kilos? Maybe. I've never seen him work out.
The momentum generators in the Guyver's limbs increase punching impact tenfold, but that doesn't mean a punch's impact equals the amount of weight you can lift.
Anyway, the Gigantic: if it boosts the Guyver's power twentyfold and Sho's bio-boosted strength enables him to deadlift five tons, in Gigantic mode he should be able to heft a hundred tons, putting him in the weight class of Marvel Comics' heavyweights.
At the low end, mind. The Hulk, when at rest, can lift about 80 tons, but his power escalates when he gets angry. Iron Man is currently in Class 100 and Thor always was - he frequently fights the Hulk and can usually fight him to a standstill. The Abomination, one of the Hulk's oldest and deadliest foes, is far stronger than him, but his power does not increase.
My point being: Thor is physically as strong as the Gigantic. I doubt he can survive a Gigasmasher blast, but then he probably wouldn't have to, since it takes time to charge and he is far more manouvreable than the Gigantic. He has more than a thousand years of combat experience, much of it from fighting huge monsters, often stronger than himself (ex. Jormungand, the Mangog, Surtur, the Celestials). All this, plus he wields Mjölnir, one of the most powerful weapons in existence.
The Gigantic is the underdog in this fight. And that's against Thor as he normally is! He has surpassed this level in the past several times, by entering a state of warrior madness, by donning his belt of power which doubles his strength, by inheriting his father's cosmic power, or by learning the wisdom of the runs.