Friend Situation?

I have two very close friends Bella and Angela. We've been planning this trip to an amusement park for a while. Angela is in charge of this trip and she invited a girl named Gertude that Bella and I don't get along with. Gertude is always mean to Bella and I whenever Angela isn't around. Angela doesn't understand why we don't want to go on the trip with Gertude and her. Should Bella and I be up front with Angela and tell her our how we feel about Gertude, or should we back out of the trip not telling Angela anything? Any advice?