Edited: I found that I did not set the path to Gimp correctly. It works.
Installed Gimp v2.8.16, set the Gimp install directory in RT Preferences to “Gimp 2” and RT could launch Gimp without any issues.
@dngimage
confirmed 
Just installed the GT3 version .
my gimp path is default windows one for 64bit OS “C:\Program Files\Gimp-2.8”
I tried “C:\Program Files\Gimp-2.8” and “C:\Program Files\Gimp-2.8\bin” , neither worked.
Got error to set my path right.
Here what it shows in option file
GimpDir=C:\Program Files\Gimp-2.8
Maybe thats the problem. In the option file, mine looks like this (double backslash):
C:\\Program Files\\GIMP 2
EDIT:
or it’s the dot (.) in your foldername.
Ok, is that the name of your gimp folder ?
Gimp 2 ??
Mine is Gimp-2.8
GimpDir=C:\Program Files\Gimp-2.8
If I try to edit it manually in options, nothing shows in preference and same error.
I am running gimp 2.8.14 , compiled by Partha
http://www.partha.com/
Yes, it is.
I’ve downloaded Gimp from the org. download page and did a default installation.
Ok, maybe that is it, its hardcoded for Gimp 2 which gimp org uses.
But it should work where every you point to, even if you renamed folder.
Weird, I thought the default Gimp install directory is “C:\Program Files\Gimp 2”, not “C:\Program Files\Gimp-2.8”. Maybe it’s specific to Partha’s build. Can you try an official version?
I guess it is partha probably changed it .
I don’t want to uninstall and use default gimp, I will lose all my extra plug-in and setup.
But I do have MV win10 I could try but I am sure it be like you said gimp 2
what do I put there, full path or just gimp-2.8.exe ?
You have to enter the full path + …exe and no double backslash
ok, I did full path
“C:\Program Files\Gimp-2.8\bin\gimp-2.8.exe”
still get error
Wow, finially , this worked “C:\Program Files\Gimp-2.8\gimp-2.8.exe”
I had to add copy of gimp-2.8.exe in there
I remember long ago, RT didn’t like if you pointed to bin folder, it wanted just root folder name.
Without being able to verify this: Is GIMP usually started from a .bat file that sets some environment variables to make it run from bin/? Maybe try starting GIMP by running Program Files\Gimp-2.8\bin\gimp-2.8.exe in a cmd shell from C:\ and see if that fails, too?
@Edward_Kiefer do "C:\Program Files\Gimp-2.8" or "C:\Program Files\Gimp-2.8\" work? Without the gimp-2.8.exe part. Please try both.
Here’s the part of options file which works fine on my machine:
[External Editor] EditorKind=1 GimpDir=C:\\Program Files\\GIMP 2 PhotoshopDir= CustomEditor=
And these two options work on my Windows 7 machine:
[External Editor] EditorKind=1 GimpDir=D:\\Portable\\GimpEval-2.9.3-samj\\64bit PhotoshopDir= CustomEditor=C:\\Program Files\\Gimp-2.9\\gimp-2.9.exe
One is for samj’s portable build, the other for installed Partha’s build.
Ok, I see why “C:\Program Files\Gimp-2.8\bin\gimp-2.8.exe” doesn’t work , there is no gimp-2.8.exe in bin, only gimp.exe and running that from command prompt or clicking it doesn’t work.
The gimp desktop shortcut is “C:\Program Files\Gimp-2.8\Gimp-2.8.exe” and of course that works.
PS: I guess I didn’t copy that exe over, I thought I might of as I thought it was a duplicate (I assumed a gimp-2.8.exe was in the bin folder with the other exe’s ) .
tried “C:\Program Files\Gimp-2.8” , no quotes >no good
tried "C:\Program Files\Gimp-2.8" no quotes, no good
only C:\Program Files\Gimp-2.8\gimp-2.8.exe works
Option files looks like this
CustomEditor=C:\ \Program Files\ \Gimp-2.8\ \gimp-2.8.exe
So it’s safe to say that the recommended way of using that is to point to the GIMP executable.
One more thing, please edit your options file and try changing
CustomEditor=C:\\Program Files\\Gimp-2.8\\gimp-2.8.exe
to
CustomEditor=C:\Program Files\Gimp-2.8\gimp-2.8.exe
and report whether it still works.
(note: your line had a space between each slash, \ \ instead of \. That was a typo, right? Your options file didn’t really have spaces there?)