Gimp Portable is a windows specific recompile of Gimp, which allows it to be used as a so called
portable app.
Rather than using the normal Windows registry to store paths and other configuration data, a "portable app" stores its settings in one or more configuration
(.ini) files located in the home directory of the app. Check the directory structure of Gimp on your USB flash drive to determine if the hidden folders for scripts, brushes and such, still exist. I suspect they are still there. You should be able to simply download/extract the scripts or brushes and place them in the associated hidden folders, then reboot. After that, you should be able to use the new scripts & brushes. This is the same install method as one would do on a non-portable installation.
It's possible the folder may have been moved in Gimp Portable so check the
.ini files on the USB device, as they may direct you to the folder locations.
If this doesn't solve your problem, you may also want to post a question on
portableapps support forum.
Of course, if it were me, I would just load up
Puppy Linux on my USB drive, boot Linux directly from the USB drive and run the full version of Gimp.