Assuming you have GIMP 2.8 installed to the default directory, you can do this via a registry script created by David L. To add the 'Edit with GIMP' option to system image types, save the following as a .reg file and run it.
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\Edit with GIMP]
[HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\Edit with GIMP\command]
@="\"C:\\Program Files\\GIMP 2\\bin\\gimp-2.8.exe\" \"%1\""
Note that the latest version of GIMP is 2.10, so
gimp-2.8.exe
above will have to be modified togimp-2.10.exe
. (Otherwise, Windows 10 will just have an annoying popup that makes you manually choose GIMP from a list.)