capodrone, if this is your first quadcopter you may want to consider flying for a little bit without the gimbal at all. P2's don't usually come with the gimbal preinstalled so simply don't install it right away. Especially if this is your first foray into the hobby (if you're already a pro just disregard the rest of this post) chances are high you'll have some sort of crash, tipover, or at least a very hard landing... almost everybody did when they were new myself included

I've had maybe a half a dozen serious crashes myself and ruined 1 gimbal. Luckily the Phantom2 itself is a pretty robust piece of equipment... the gimbal not so much it's way more delicate.
So if you're at an elevated risk of some sort of crash, spend some time... at least a few flights or couple days flying around WITHOUT the gimbal attached to get a feel for it at short, visual range. Once you've gotten a good feel for flying the quad (and possibly a little hot dogging out of your system) then attach the gimbal and gopro.
You can absolutely update the P2 firmware without the gimbal attached, and then again once you install the gimbal it'll just spot the new hardware and apply any update to the gimble firmware separately.
on the other topic, a GoPro BLACK (3 or 4) is vastly better than the cameras on the Vision series (yes, even the Vision+). the supported resolutions and framerates it captures are way higher. There's also the fact that the Vision series cameras are single-use only on the Phantom. A GoPro you can detach and use for anything at all.
That argument is in fact negated on the Inspire 1 where you can detach the camera and use it handheld... and even though the Inspire camera captures 4K some guys around here with more knowledge than me about video are still saying the gopro is a better camera. Let's also not forget the price tag on an Inspire $2900! a P2+Zenmuse+GoPRoBlack+FPV still is only around $17-1800.
So yes, a P2+Zenmuse H3-3D, if you factor in a good FPV system and especially if you include the cost of a GoPro is more expensive in the end than the Vision series, but you've got a much more powerful platform than a Vision... better camera, better FPV range.