hang on... no matter what else I want to make sure you know to NOT have your GoPro's wifi on during flight. The RC operates on similar frequencies as wifi so if you have the gopro's wifi turn on, you run a serious risk of the gopro's signal overpowering the RC and your Phantom flies away and is lost.
so definitely don't do that.
I only have a H3-2D gimbal but for starters when configured properly I always had a preview in the gopro app. of course you must connect your device (ipad, iphone, whatever) to the gopro's wifi, and then the app can access the gopro to change the settings, preview, etc. I used this setup ONLY ON THE GROUND to configure the gopro, but always turn the wifi off immediately.
so I'm not sure why you're not getting a preview... but I can tell you that indeed the Gopro passes the video signal to the gimbal (which can then be tapped into for FPV) at any resolution... I made recordings in every possible resolution including 4K and it always worked fine.
Sometimes the FPV signal you broadcast can look strange and squished (when the resolution doesn't perfectly match your display) but the actual recordings direct on the gopro are perfectly fine