I had the walkera g2d gimbal for a while powered by the aux wires coming from the landing gear. Then I finally took it apart to attach the pitch control.
I have read many times that the f2 alone is enough to power the gimbal, but I just left my aux wired up anyway as well as plugged in the f2. I have went on many, many flights now with this setup and it works great. Every once in a while the gimbal will also spaz out and go almost vertical for a second, but that happened on my last cheap chinese gimbal frequently. I just assume these cheap gimbals are very glitchy, but it's never affected my phantoms flight.
Honestly, I think that using a second battery to power your gimbal is not worth it. I have never tested it myself because i've never bought a small battery, however I still think the added weight is less advantageous. I've used my p1 with p2 props both with just a gopro hero 3 with the case on, as well as with the walkera gimbal with gopro3. The battery life is the same, even with the gimbal wired up to the quad battery.