Gallus is a huge man of amazing size and fit standing seven feet tall and was discovered as a slave in a quarry in Britain and brought to the slave market where interest was indeed high among the buyers some for a strong laborer, some it was a novelty and others looking at him as a gladiator. The fact he outsizes any human in the arena and add to that he is balanced and fit with a good head on his ample shoulders is a benefit. Oddly he sees being a likely gladiator a good chance at freedom if he can learn to be a good one and is eager to learn it, his sheer bulk and size is also in his favor it clearly will take much to kill him. The only issue amusing is arming him with the right weapons and affording the armor nothing laying around will be of much benefit. But he should please the crowd and could be a champion he has all the merits for that one should have. It should be noted he has a level nature his years as a working slave taught him to obey, work hard and learn to be of value as can be seen by not having been whipped and so far.
He is well endowed with a nice foot long weapon as well suited to his size and should please any ladies seeking his services as well.
His training will be a challenge also his size makes normal combat practice a bit impractical and his strength his size will mean having a training staff able to adapt and exploit it, just not any Dominus would be likely to do so and afford such a slave. A reason his current slave trader is clearly going to ignore lesser patrons.I would think you need all kinds of gladiators and the odd giant human would be amusing, unless he won't fit the game. This is just my idea for a male gladiator.