Thanks for sharing your GIMP tips! 
I was also happy to hear that you also use digiKam, as I do. 

When I do not want to edit with digiKam itself, I can right click on the thumbnail in digiKam to get a menu where there is an option "Open with", where I have GIMP, RawTherapee and more softwares listed. 

Furthermore, I have added in the Main tool bar the button "Open with predefined sw", which in my case is RawTherapee. 

Remark: I'm not sure I've translated the digiKam terms correctly into English. 