Gwen Tennyson (Classic)/Omniverse flashbacks

In Ben Again, Gwen was intrigued by the sudden increase in maturity of the 11-year-old Ben, whose mind had been switched with that of 16-year-old Ben. Eventually, she helped to defeat Eon and her memory was erased about the events of the episode except for a thing that 16-year-old Ben in 11-year-old Ben's body told her in a vague memory. 
In Mud is Thicker Than Water, she was constantly annoyed by Lucy messing with her and threatened Ben.
In Evil's Encore and From Hedorium to Eternity she helped to defeat Dr. Animo and Zs'Skayr respectively.