for starters Damon is of course correct - a GoPro does indeed have an "idle turn off timer" where if you leave it on, it will shut itself off automatically after X minutes to save battery, and this setting can be disabled so that does not happen.
Furthermore, that only applies when the GoPro is NOT recording. As long as you hit the record button and are recording your flights the gopro will never just turn off, unless perhaps it fills the SD card and stops recording, AND you have the sleep timer enabled. but honestly get yourself a couple of 16GB or 32GB SD cards, keep them clean so one doesn't fill up mid-flight and record all your flights. and disable the gopro sleep timer.
Eric, almost nobody does a separate FPV camera. I've seen a few guys yes... but I literally mean like 2 ever... extremely rare cases for some specific purpose. I don't know where you got told that the gopro isn't a good or reliable FPV camera, as far as I'm concerned the whole product is designed to use the gopro for that purpose, it works wonderfully so I don't see any point in adding additional weight and equipment. Besides, if you're recording videos with the gopro don't you want to see what you're recording?
The Zenmuse gimbals bring the video signal back up to the Phantom mainboard specifically to use it as a live FPV feed... all you need is an appropriate PnP cable to get that signal, link it through the iOSD Mini, and then out to a video transmitter. As mentioned earlier there are theoretically some things that if they all lined up perfectly, could cause the gopro to shut off during flight. Disabling the sleep timer on the gopro, making sure your SD cards have enough room to record the entire flight... some VERY basic things you should be doing anyway will prevent any problems of that nature. Don't forget there's always the Failsafe option which will bring the Phantom back home where you can switch to Line of Sight flying to bring it in to land. This happened to me once, I was running my GoPro on battery power (not powering it from the Phantom) and the battery died... poof no FPV. but no big deal... I flipped Failsafe, the Phantom flew back overhead and then I resumed control to land. easy!
The iOSD Mini does a lot more than show your height, we refer to the entire suite of information it gives you as "flight telemetry data". Altitude, distance from home point, compass heading, climb/descend rate, lateral velocity, battery level, flight computer mode (GPS, ATTi, Failsafe)... all of this is extremely valuable info when you're talking about FPV flight with a Phantom.
The only reason I would consider an additional camera is that it seems that it may be difficult to fly the quad when everything looks stable and level from the gimbal... That is what I meant by "does the mini iosd show the attitude" not altitude. I guess I just need to buy one...somebody tell my wife that I need a Phantom 
