You're right in that 800 years is plenty of time for things to change radically. It's really not possible to predict how the hellenistic kingdoms would have progressed without Roman intervension. Also the Gauls may be able to survive since there would be no Caesar to conquer them. But would they be able to resist pressure from germanic peoples long enough to forge a strong, solid kingdom?

Regarding the success of the islamic expansion, it really comes down to the political situation in the lands bordering Arabia. With the Byzantines and the Sassanid Persians having bled each other dry, there was a perfect opportunity for conquest. The outcome may have been very different had the invaders faced strong, vigorous states instead.