I have the original model of the GPD Pocket, bought in 2017. Tried many Linux distros before until installing Ubuntu Mate definitely for some years now. Recently I bought a USB Type C to Displayport cable, in order to use an external monitor with the laptop. Originally didn't worked, so I had to downgrade the BIOS from the latest version to the unlocked one (dated June 28th, 2017, as seen here), enabled the DP Over Type C option.
At first it worked, I could see the boot screen on the external monitor, but soon after it stopped when the system loaded. Connecting the cable after the system booted also doesn't work. Is there any work being made to support Displayport over Type C on the GPD Pocket? I believe that is an old issue still opened on other distros, none of them as organized as Ubuntu Mate.