for a Phantom you want the DJI iOSD Mini, it's tiny and light. basically it sits inbetween the video signal (from the gimbal / Phantom main board) and the video transmitter and overlays telemetry info onto the screen... very useful information such as:
horizon bar (so you can see the tilt)
home point indicator
battery level
# of GPS satellites locked
height (from home point)
distance (from home point)
climb or descent rate (up/down speed)
lateral movement rate (forward/back/side speed)
and a few other little things such as your flight mode (GPS, ATTI, etc)
The iOSD Mini connects to the CAN-BUS port on the Phantom - watch the first 1-2 mins of
this video it will show you how to move the CAN-BUS connector up into the body of the Phantom so you can plug the iOSD mini up in the shell and not have wires hanging around. The rest of the video is useful, but remember the parts about soldering cables does not apply if you simply buy that pre-made cable I linked before.
If you don't install one you will see ONLY the video feed from the gopro and when you're flying FPV it's easy to not watch the phantom to see for example the low battery warning. with the iOSD you have all the flight info you need right there on the screen, like a jet fighter or something
the image below is a screen grab from the iOSD Assistant sofware, in the picture with the sky/trees all the WHITE stuff is coming from the iOSD mini overlay
