Looks very sketchy but I’m not to sure if it really is GIMP. It’s code is old and complex changing it to look like in the screenshot would be a big project on itself.

I ran Debian on my Pinebook Pro until it tied about 6 month ago. Was painfully slow but it did kind of work.

Stuff from the repository of your distribution generally can be considered save but everything involving a third party might not be.

This counts for both other Apt repositories as well as Flatpak. You likely have Flathub as an Flatpak source and while they have some checks and controll instances it is possible for untrusted third parties to upload packages including non-free ones there. I do not now of any incidents but some suspicion for packages with full system access can’t harm.