Gandy defeated Fran Halsall with a time of 57.25 seconds, beating Halsall's 2010 time by 0.15 seconds.
"It's the greatest feeling ever," said Gandy. "All I've wanted to do for the last four years is be able to say 'I'm going to the London Olympic Games' and now I can say it. I am just over the moon."
Third-placed Jemma Lowe was the unfortunate athlete to miss out on a place at the Games.