While both Darktable and RawTherapee are available as standalone flatpaks, neither one provides functionality needed by GIMP to be able to open CR3 and other raw formats.

This results in an error:

> GIMP Message
> 
> Opening '/var/home/[FILEPATH].CR3' failed: There is no RAW loader installed to open 'Raw Canon' files.
> 
> GIMP currently supports these RAW loaders:
> - darktable (http://www.darktable.org/), at least 1.7
> - RawTherapee (http://rawtherapee.com/), at least 5.2
>  
> Please install one of them in order to load RAW files.

See discussion on AskFedora https://discussion.fedoraproject.org/t/flatpak-gimp-there-is-no-raw-loader-installed/148375/1

And request against Flathub: 
https://discourse.flathub.org/t/darktable-rawtherapee-raw-loaders-for-gimp/9333