media-gfx/digikam-7.5.0 builds fine with media-gfx/graphicsmagick-1.3.37 instead of media-gfx/imagemagick.
In /etc/portage/profile/package.provided I have media-gfx/imagemagick-7. I have media-gfx/graphicsmagick-1.3.37 installed with USE=imagemagick. media-gfx/digikam-7.5.0 builds and runs without any apparent issues.
I suggest to change the dependencies of media-gfx/digikam from media-gfx/imagemagick:= to:
*** Bug 842480 has been marked as a duplicate of this bug. ***
I don't think it's that simple. I think it uses 'convert' and can indeed work with GM or IM for that, but it optionally can use IM libraries too. I don't think it can use GM's.
A lame fix here might be to just always RDEPEND on virtual/imagemagick-tools but make USE=imagemagick for using the libraries.