Actually it would be possible for pretty much any soldiers to win in that situation not only the gladiator champions.
Roman shields look flawless against arrows or such but in close combat against mobile soldiers they make you just a dead weight.
First of all for a functional shield wall 5 sides of a box formation must be tightly connected(some later shields actually had hooks on them for that purpose)...in this case 8 people are just too few making them very unstable.
secondly big square shields diminish your offensive capabilities a lot,it leaves you a small space to hit with your sword and you can't see much.In large battles you just stab directly in front of you and hope you will hit something.
Now imagine yourself holding large heavy shield with your left forearm,you put the pivot point in the middle of the shield,if someone kicks on the upper part of the shield you will get hit in the head by our own shield xD
Now imagine if someone jumps on it,you will be lying on the floor under your own shield...if you are lucky enough and they are badly connected you get the domino effect for extra fun.
So if 4 gladiators just jump on the shields you instantly disable 4 romans,leaving 4 of them standing...now if those 4 keep their formation looking forward they are dead because their left sides are defenseless,if they do turn you can now jump on them too...now you have 8 romans lying on the floor squirming like the turtles in less than 10 seconds.
In similar way that's why rome lost so many small battles against unpredictable "guerrilla" armies and won so much in large battles and sieges.