Defining GIMP 2.10.28 to Scribus 1.5.7

[rich2005]

I think not possible. Since both Gimp and Scribus are flatpak versions they are both sandboxed and "do not see" other applications. 

When you flatpak list in a terminal you get this

   

The command to run Gimp is flatpak run org.gimp.GIMP  but put that into Scribus external applications and it gives a error "missing or not installed".