This is something only the product's tech support can answer. They can use any of the format's features that doesn't noticeably affect playback to secretly mark a file. You signed up for this by using software that is tied to a product. Below is what they let on.
If you give a sample of a valid file, we might be able to see anything unusual about it.
Here's all the GoPro's support says:
Video files are recorded using the h.264 codec and the MP4 file type,
the exception being some resolution/frame rate combinations on the
HERO6 Black. With the HERO6 Black, HEVC (h.265) is utilized for
applicable modes. For more information, see: HEVC Explained.
Step 1: View and Trim
The following formats can be imported into “Step 1: View & Trim” and
converted into GoPro CineForm files for use in “Step 2: Edit”. You can
use GoPro App for Desktop to open these files in Step 1 as well.
MP4 video files from the original HD Hero Original, HD Hero 2, HERO3,
HERO3+, HERO and HERO4 cameras. JPG files from GoPro HD Hero, Hero 2
and Hero 3 cameras (.jpgs from other sources may work, but have not
been tested nor are they officially supported)
Step 2: Edit
Step 2: Edit can import the following file formats:
Video formats: GoPro CineForm AVI or MOV files (MOV only on Mac)
Audio formats: MP3, WAV, AIFF, M4A and CAF