Also of note was the talented (if small for the moder HW era) Horace Notice, same era. Think Mason should have been let off the leash earlier perhaps, definitely a handful for any era, immensely strong, heavy handed, and in the modern age, media friendly.
Mason unfortunately was always in Bruno´s shadow. Surprisingly put punches well together but was slow afoot, very strong and seemed to have a good chin though never fought any punchers till Lewis. Took everything from Lewis (especially the jab), fought bravely and even though in obvious pain from the eye injury let loose a final barrage at Lennox with no luck. The over the hill Biggs and showed his limitations at world class but he was a tough nights work for anyone. His ko´s of Jess Harding, Denroy Bryan and Lorenzo Boyd are spectacular.
Heard he was better than many in the era just ending Chisora, Joshua, Whyte, Gary Cornish and previous era including Matt Skelton, Danny Willlams, Big Audley and John Fury.
Gary went under the radar a bit while he was fighting. He was one of the first super-heavyweights coming in at 18 stone and more and although he wasn't the quickest mover he had fast hands. He definitely suffered from being in Bruno's shadow and it's difficult to judge him by modern standards because there are more decent British heavies now than back then; in those days you had Bruno then afterwards came Lennox Lewis, with Gary as number two/three to them, then there was a big drop to domestic level. As TED822 said, he'd have beaten most of the ones that came afterwards.