There is no natural way for a gorilla to be on the island.
When the professor is cutting bamboo, the bamboo pole bends at the cut before he draws the saw through it. The saw never goes clear through the pole, but a moment later, it's a very clean cut.
The second time the shadow of the gorilla comes over Gilligan and the skipper asleep in the hammocks it shows over Gilligan but not the skipper.