If Gladiator can take all of earth's heroes that were their against the Void in the last one shot, then he could take Sentry. Thor, Thing, Namor, Doc Samson, Dr. Strange, Reed, Hulk, X-men, and a whole bunch of other heroes, then he could take Sentry. Earth's heroes would have gotten killed, and earth swallowed in a void of darkness if Sentry didn't make himself forget. All of those who knew of the Void were scared sh!tless. Even villains like Doc Ock came out to battle the Void. Sentry will defeat Gladiator.