Plan to buy Flexify drone and use iPhone to receive video streaming, but the Flexify does not support iPhone. Could I buy a Gimbal that has iPhone support to accomplish the work? I plan to buy a 3DR Gimbal (does support third party transmitter) and connect to Flexify TX Video Module. if the configuration is correct, according to 3DR Gimbal manual, need to change to channel 6 to talk to the third party transmitter, could I use the 3DR iphone app to receive video stream. the other thing I need to do is to use a converter USB type C to Mini USB to connect the Gimbal to the new GoPro 5 session since the 5 session has type C USB connector and is not currently available in 3DR Gimbal.
I would stay away from using wifi to send video from your drone as firstly you will not get good range, and second it can interfere with the radio control signal that runs on same 2.4Ghz. Drone like DJI use a different frequency for radio control and another for video.
So you will be best using a FPV video transmitter connected to your gopro. And also there are many cheaper gimbals that work well as again the 3DR gimbla is built for use on the solo and not designed to be used on other drones.
I am not familiar with the flexify drone, but if it already has built in FPV transmitter then that is great. For gimbals I can personally suggest the Z1-Tiny2 form zhiyun tech is probably the best you can get in terms of price and performance, however I do not know if it will directly fit onto your drone, so there might be abit of modifications required. We also sell the Z1 tiny 2 gimbal, but our shop is based in the UK (we do ship worldwide though)