How do I mount a GoPro Hero 7 via USB in Ubuntu 18.04?
I had the same issue when I used USB-C to USB-C cable. Once I switched to USB 3.0 - USB-C cable everything was back to normal. I also had gMTP installed, but I am not sure if it had any impact.
Pressing the Settings button on the side of the camera made it visible to my Linux system.
Here is what the link to my GoPro folder looks like, if this helps in any way:
mtp://GoPro_HERO7_C3281328889740/GoPro MTP Client Disk Volume/DCIM/100GOPRO/
For GoPro Hero 8, go to Preferences → Connections → USB Connection → MTP.