Becoming a content creator on GabTV:
If you are brand new to GabTV and this is your first video as a Gab Pro subscriber:
When you *first access GabTV,  select the log out button then log in to update your GabTV account and allow the system to create your new GabTV channel.
This is a known issue being addressed.  (03/02/21)

Video Upload issues:
First, how big is the file? 
Just in file size, how big is it? If the file is bigger than 4 GB/3 hours/day (which ever comes first), it will fail.  *May vary during site construction.

Next, how long is the video (time)? If longer than 3 hours, it will fail.

After that, the only reason the file would fail is if it's corrupt in some way or can't be read by FFMPEG.

If it is an MP4 DASH file, you may need to transcode to a more typical video file.
Please use regular MP4 files with h.264 or h.265 as their video codec in standard streams (DASH files are segmented).

Generally, the best results come from 1920x1080 videos in h.264 video at 4-6Mbps with AAC audio at 160Kbps-320Kbps is best. 
Edit 7/21 Files need to be less than 4 GB in size and 3 hours in duration.
*(these limits are subject to change)