Sometimes the packager of the snap package gives the snap package explicit permission to access external media and sometimes the packager does not, so the decision to give the snap package explicit permission to access external media is left to the user. Since there's no overriding policy, the decision to enable removable media support is up to the packager of the snap package and after that it's up to the user.
I have observed that snap packages are generally less reliable than apt packages which means that I often install a snap package, and then I have to uninstall it because it didn't work properly. So I guess that not enabling removable media support on some snap packages adds another layer of security to the snap package that the packager thought it needed.
If you need GIMP to have full access to external media (such as USB flash drive, SD/MicroSD card, additional mounted hard drive and so on), run the following command:
sudo snap connect gimp:removable-media
$ sudo snap connect gimp:removable-media
$ snap connections gimp
Interface Plug Slot Notes
content[gtk-3-themes] gimp:gtk-3-themes gtk-common-themes:gtk-3-themes -
content[icon-themes] gimp:icon-themes gtk-common-themes:icon-themes -
content[sound-themes] gimp:sound-themes gtk-common-themes:sound-themes -
cups-control gimp:cups-control - -
dbus - gimp:dbus-gimp -
desktop gimp:desktop :desktop -
desktop-legacy gimp:desktop-legacy :desktop-legacy -
gsettings gimp:gsettings :gsettings -
home gimp:home :home -
network gimp:network :network -
opengl gimp:opengl :opengl -
removable-media gimp:removable-media :removable-media manual
unity7 gimp:unity7 :unity7 -
wayland gimp:wayland :wayland -
x11 gimp:x11 :x11 -
There is also a GUI way to give a snap application access to removable media from the Software app. Snap packages that allow access to removable media will display a Permissions button in the Software app after they are installed.
Click the permissions button and then toggle the Read/write files on removable storage devices slider from Off to On as shown in the above screenshot.