You don't know what terminal is, yet managed to try the code in Terminal. Interesting.
Anyways, it partly depends on what version you have of items. I'm running X11 2.1.6 and Gimp 2.6.6. For the mouse focus it looks like the property in X11 is wm_ffm (for my X11 anyways), which is essentially the same as the one I gave before, but simply has been renamed by this version. So that will change the command to,
Code:
defaults write org.x.x11 wm_ffm -bool YES
This file we're editing with this command is located at
~/Library/Preferences/org.x.x11.plist and can be opened with Property List Editor. (The ~ represents your home directory.) You can check the box for the above property using the Property List Editor app if you prefer that over using Terminal. Just be sure to restart Gimp afterward for the change to take effect.