Rescue from Gilligan's Island (1978) is the gang finally reaching civilization and being miserable from being 15 years out of touch. Mary Ann reunites with her long time fiancé even though the series made it clear she didn't have a boyfriend, go figure.

The Castaways on Gilligan's Island (1979) The gang is rescued again, but they decide to stay and turn the island into a resort, why not? This was meant as a tv pilot to try and cash in the "Love Boat" and "Fantasy Island" popularity at the time. Thankfully it didn't take.

The Harlem Globetrotters on Gilligan's Island (1981) is the least sucky of the reunion films. Why you might ask? Well, I got FIVE REASONS for your ONE QUESTION! The first...
...
it's kinda personal.

Second reason: Martin Landau, automatic FTW, as the main villain.
Third reason: Barbara Bain as the other main villain.
Fourth reason: Scatman Crothers
Fifth reason: This film contains many of the original Globetrotters as seen from the animated "The Super Globetrotters" and in TRUE Harlem Globetrotter's fashion, they fight robots in a basketball game.

Tina Louise isn't in any of these films. That's a good thing.