Gretchen apparently serves as a second-in-command to Isabella in the troop. In the instances where Isabella does not actually listen to what Phineas is saying to her due to drifting off into fantasies, Gretchen repeats it for her convenience. ("Isabella and the Temple of Sap")
At first glance, it may be difficult to tell which of the Fireside Girls is Gretchen. Early on in the episode, she is the girl who asks the question about Phineas' racecar being electronically fuel injected; however, she is not given a name at this point. Her name comes later when Isabella compliments her on the bow she ties on the car. The last piece to the puzzle is that Ariel Winter is credited as Gretchen in the end credits for the episode. Therefore, she has a speaking part in the episode, and must be the Fireside Girl who asks the question. The episode's print novelization in Speed Demons further confirms this by way of the narration explicitly stating that it is Gretchen who asks the question.