Note: If you installed both the stable and beta repositories, the desktop (menus, etc.) will see only one version at a time. To make sure your desktop sees the development version, run this command:
flatpak make-current --user org.gimp.GIMP beta
Or respectively to restore the stable version as the visible GIMP application:
flatpak make-current --user org.gimp.GIMP stable
You may also create shortcuts running specifically one of the other version.