The television star's donation - reported in local press to be nearly £3,000 - came after he heard about the campaign for the Parrot in Carmarthen.
Gilbert, who was born in the town, contacted the West Wales Music Collective and offered to push the appeal to the halfway mark - £5,000.
He said the town needed live venues.
He said if Carmarthen had had them when he grew up, he would have probably got into stand-up comedy sooner than he did.
When asked by BBC Radio 1 if he was the "mystery benefactor", Gilbert said: "I am not a mystery benefactor any more then...
"My old home town, Carmarthen in west Wales, there is only one music venue, I think I am right in saying, called the Parrot that closed down for one reason or another and now there is a lovely group of artists and musicians -  local do-gooders - who are trying to get enough money to reopen it and improve the place. They are a non-profit type of thing.
"A town like that, in west Wales, a town like that as in a small town, needs music venues, needs comedy venues, need venues, live performance venues... People need them."
Gilbert, who now hosts comedy music quiz Never Mind the Buzzcocks and presents a show on BBC Radio Wales, added: "It took me 33 years to do my first stand-up gig, I hadn't been to a comedy club until I was 27 years old.
"Had we had them in that small town I would have gone.
"I think it's the least you can do is put a bit back in."
A spokesman for the West Wales Music Collective told the FYI: Carmarthen website that it was now hopeful of reaching its £10,000 target.
A mystery benefactor who gave money to help reopen a live music venue in Carmarthenshire has been revealed to be comedian Rhod Gilbert.