In the context menu for an image file (e.g. JPEG, TIFF, PNG, etc.)
in the Finder, in the "Open With" menu item, Gimp is not listed as
an option. I think this is because Gimp just has the wildcard
document type defined in it's Info.plist.
In order for Gimp to appear in that menu, it probably needs to
define specific entries in the CFBundleDocumentTypes key of the
for JPEG files.
That way, Gimp will appear in the "Open With" context menu item's
sub menu for the defined types.
I'd suggest to add entries for the standard image types known to
be recognized by Gimp (JPEG, TIFF, PNG, BMP, PDF, etc.) as well as
Gimp's own document type.
(Keep the wildcard document type in addition to allow dropping of
any file onto Gimp).
[This is for Gimp 2.0.3-2]