'Galley FM', and I presume Radio Galley means the same thing, is essetially all the stories, rumours and general sh*t that crew go on about in the galley -
eg "Sally I heard on galleyFM that you and Scott broke up because he was caught shagging the supervisor in crew rest and the captain came down and busted them but ended up joining in and now he is getting divorced and his wife is suing him for everything they own, even though she had secretly shagged Scott on 4 previous occasions anyway, and the captain had no idea"
(all said, of course, in the best Vicky Pollard voice you can come up with
)
The true story there may well have been that Sally and Scott broke up quite amicably and are still close friends!