With the knowledge we have now, it shouldn't be hard to decode the video transferred by USB, ie. using Pi Zero. And telemetry it trivial - we know all the packets.
So it's very hackable, just not zero effort - you'd have to reverse engineer video decoder binary (it has symbols) and make your own decoder based on that. The video pipeline in GL300a RC is symmetrical to AC - by which I mean the video going into DaVinci in GL300a is in the same format that the video going out of gimbal board in Ph3 Pro.
In other words, if you connect the HDMI Module from GL300 to the gimbal output, you should get proper video at HDMI. As long as you connect it properly.