Your brushes, gradients, and scripts are stored inside: ...\GIMPPortable\Data\.gimp-2.10\ If you want to upgrade to a newer portable version, to the new version’s Data directory. You will lose zero custom settings.
While "zip" is often used as a general term for compressed software, GIMP Portable is most commonly distributed as a self-extracting .paf.exe file via PortableApps.com . gimp portable zip