I get the same error message if I try and open an image file in Nautilus from the USB drive by Right-Click image file > Open with GIMP
I installed GIMP 2.10 with snap:
$ sudo snap install gimp
There's obviously something I'm missing in terms of configuration, but I can't find a similar question posted yet.
I was pointed to the solution to resolve the problem:
How do I get the latest GIMP version available?
You need to give the snap GIMP package explicit permission to access external media once installed:
sudo snap connect gimp:removable-media
Why we have to do this extra step, is eluded to by N0rbert:
How to get access to USB-storage from the application installed as Snap?
"...if the application does not have auto-connect of removable-media we need to connect it manually"
My question is why would an application package be created without the ability to access external media by default?