What was Gilligan's first name on "Gilligan's Island"?

Gilligan's first name was never revealed on the show. There is some belief Bob Denver announced on a talk show years later that the creators of Gilligan's Island had said Gilligan's first name was Willie.

A few events in the early 1990s spurred the claim that Gilligan was a surname, and that the character's first name was "Willy": TBS finally aired the pilot of Gilligan's Island (an episode that had never been broadcast) in 1992, and TV Guide announced in 1993 that it had discovered (from an early press release) that Gilligan's first name was supposed to be "Willy."

The character Gilligan did not have a first name

Gilligan's full name has been a subject of debate among fans of the series for decades, a debate resolved by the release of Gilligan's Island — The Complete First Season on DVD in 2004. In a documentary called "Before the Three Hour Tour", excerpts from Sherwood Schwartz's original treatment for the series reveal that Gilligan's full name was Willy Gilligan, a name which Bob Denver had sometimes given during talk show interviews. Bob Denver stated on KDKA radio interview in May 1989, that Gil Eggan was his choice for the character's name. Denver reasoned that since everyone yelled at him, it came out as Gilligan. However, no first name is used in any of the three seasons, films or animated series. The documentary also reveals that Schwartz found the name "Gilligan" by opening the phone book to a random page.

https://en.wikipedia.org/wiki/Gilligan_(Gilligan%27s_Island)#Name