Thanks for the follow up @pavlos_kairis . I did check the SHA checksum after downloading but that's not what I meant.
I meant how do you actually build the components but I think I sort of found the answer: Ubuntu MATE 21.10 for the GPD Pocket 3
@Wimpy who is a trusted member here published the assembled installation, previously I was under the impression that the people from GPD published the image which I would consider problematic.
Anyway - is there some sort of github repository I can have a look at?