Looks like there are two GIMP packages that differ only in case (gimp.gimp and Gimp.Gimp), and this seems to be preventing winget from doing anything with them: C:\Users\->winget show gimp Multiple ...
Sorry if this has been reported previously. I tried to install Gimp in an Ubuntu WSL2 shell and then in a Powershell Admin shell. Must I turn off Windows Defender Virus check to install some apps?