Similarly to our Flatpak, GIMP Snap uses a GNOME runtime, which contains a base of libraries, some of which are dependencies of GIMP. The runtime version is determined by the base: snap version in snapcraft.yaml. It is recommended to update that key value following Ubuntu release cycle.
Other GIMP dependencies which are not available in the GNOME runtime snap should be listed in build-packages: and/or stage-packages: keys if gimp-meson-log.tar artifact shows as necessary. You can take a look at the deps-debian job from the in .gitlab-ci.yml as reference.
On the other hand, if we increased the base version in particular (so as consequence the GNOME runtime too), some packages may no longer be necessary and can be removed from our manifest.

You can find the installed dependencies by running:

ls "$(echo /snap/gnome*-sdk/current/usr/lib/$(gcc -print-multiarch)/pkgconfig)"

Versioning the snap

Aside from (un)commenting and bumping babl and gegl source-tag and setting the version of gimp, we:

For a nightly build, set "devel" on "grade" and use "experimental" on build-id at gimp part "meson-parameters".
For a new development series, set "stable" on "grade" and use "preview" on build-id at gimp part "meson-parameters".
For a new stable series, set "stable" on "grade" and use "latest" on build-id at gimp part "meson-parameters".