Comment 3 for bug 1799542

you need to connect the removable-media interface (for security reasons this interface is not auto-connected), either by going to the software center, finding gimp in there and clicking the "Permissions" button where you then can allow access to "removable storage devices" or via the command line with:

sudo snap connect gimp:removable-media

after this you should be able to also drag and drop files from your external disks.