Yeah the only way you will get a gimbal working with the case attached is to basically make your own with an Alexmos board and your own brackets.... doable but not at all easy. The other issue is that the shear size of with case would mean it probably won't fit under the Phantom without massive legs and the weight won't do flight times any good at all either.
Over time in this hobby I can assure you that you will want a Hero 3 or 4... some great deals out there on the Hero 3 (the first version before the Hero 3+) and it is still a **** good camera which fits in all the current gimbals.