The so-called "Roman" thumbs down (kill him) or thumbs up (let him live) gesture to gladiators is actually an invention of Hollywood. It is true that there were gestures used in the Colliseum that denoted these meanings, but nobody knows what those gestures actually were - it wasn't recorded.
By the way, Romans also used to nod up for "no", down for "yes". They did not shake their heads for "no". Very confusing!