Thanks, Tom! Working through the child theme option I actually became aware of the
appearance > editor > 404 template
section, which I haven’t used in the past. I am not sure if this is any way connected to the child theme, but I strongly suspect this was to have been there before. In that case it would have been perfectly fine for me to simply edit the 404 page through the regular GP theme.
I am just adding this here to help those looking for the same information after me 😉
Best wishes,
Markus