I use KDE (kubuntu 24.04 / Gimp 3.0.6 appimage) so obviously some differences. I can not see anything really wrong with your .desktop file. Just for comparison this is mine.
A long shot - do you have this folder ~/.local/share/appimagekit/ and what does it contain? A zero length text file (title only) no_desktopintegration prevents an appimage making a desktop file.
Code:
[Desktop Entry]
Comment=
Exec=/media/rich/DATA1/portable/gimp3/GIMP-3.0.6-x86_64.AppImage
Icon=/home/rich/icons/gimp.png
Name=Gimp-3.0
NoDisplay=false
Path=
StartupNotify=true
Terminal=false
TerminalOptions=
Type=Application
X-KDE-SubstituteUID=false
X-KDE-Username=A long shot - do you have this folder ~/.local/share/appimagekit/ and what does it contain? A zero length text file (title only) no_desktopintegration prevents an appimage making a desktop file.