Brooke didn't love Gary
I think that's the point that a lot of people miss at the end of the movie. Gary told Brooke that he loved her and she told him "I don't feel the same way" so she didn't really love him.
A lot of people gave examples of why she didn't really love him because when he was beat up by her brother, instead of feeling bad she felt a bit happy. She also did a lot of cold hearten things like voting him out of the team that he loved and bringing in dates. I think she was simply obsessed with having him as her boyfriend because they had been together for so long, but I don't think she loved him at all. I thin she got used to him but doesn't mean she loves him.