This is making me sad now. I feel bad for Rainbow Dash that she had to discover that her best filly friend has changed and become a bully. Must have been hard for her, but luckily she has really good friends. Would be in interesting to see an episode in Season 4 in which it's showed how Gilda was in the past and explained how she became the bully we know from Season 1.
Gilda needs to return for Season 4. Seriously. There are thousands of storylines you could run, from their days at the academy to the real author of Daring Do to an ambassador trip to the Griffon Kingdom.