When Katniss returns home after the Games (first book still), her mother tells her that her "cousin" is waiting, and then points out Gale; they explain the whole thing to her then, and then it's a pretty minor offhand detail throughout the second book.
Of course, if Gale's so insignificant in the movie, that would make absolutely no sense whatsoever to the movie-goer who hasn't read the book.
Of course, if Gale's so insignificant in the movie, that would make absolutely no sense whatsoever to the movie-goer who hasn't read the book.