Still tho, Gregor is not just some commoner. He spent his entire life as a warrior, surrounded by professional warriors and people killing for a living. He's not just some random overly large peasant who's been conscripted, given a weapon and told to go to war. He's been on the front lines for decades and when he wasn't, he was swinging swords with the pros in the courtyard or smashing skulls in taverns or god knows where.
Do anything for that long, even if it is raping, pillaging and brutally murdering people, and you will get good at it. The particular skillet he developed may not be based on finesse or speed and he's most definitely not master like say Jaime at the sword or Loras with a mace but you can bet he knows what he's doing. Huge and aggressive is scary enough as it is; add to it a modicum of competence and you can picture why everyone is scared shitless of facing the guy.