Time after time, Gilligan discovers something on the island or is the first to spot a wild animal or stranger on the island. Each and every time, the skipper doesn't believe Gilligan.

I was wondering why, since Gilligan may report something a foul, but he doesn't lie. He may believe he sees something like a sea creature or something, but he usually sees something worth investigating.

Perhaps Gilligan reports more than is in the episodes.

He did attack a weather balloon when he thought is was a monster on the island.

You're right, if one of the other castaways saw the same things that Gilligan did, they would be taken more seriously. He does however get things confused pretty often. Maybe that's why everyone questions his sightings and stories.