The Zenmuse H3-3D was designed specifically for use with the Phantom 2, as such I don't really see any reason to run another gimbal unless cost is a major factor for you, I think you can get cheap, non-Zenmuse gimbals that work on a P2 in the low to mid $100's range.... but if you're patient enough I bought a perfectly functional, what appears to be brand new H3-3D with a scuffed up, opened to verify components BOX (the gimbal was in new condition) on Ebay for about $190US. They easily go for the low $200's, I spent about 3-4 weeks of close watching on all Zenmuse gimbal auctions before I was able to snag one at that price. My point is you DEFINITELY shouldn't spend retail ($379) for a H3-3D if you just want to buy one by itself and save some money.
Make sure to get the Phantom 2 version though, there IS another version with a controller that isn't necessary for P2's.