I have extracted the compiled files from GAP from the GIMP installer site: http://gimp-win.sourceforge.net/
I have compressed the exe files using UPX in accordance with https://portableapps.com/support/gimp_portable#modifications
Finally I put them in a zip file that corresponds to the GIMP portable file structure.
The file is available here: http://5011.com/portable/PortableGAP.zip
Just unzip the file into the root directory of your GIMP portable installation and you'll have the GAP part there as well.
Report any issues found on this forum and I'll try to follow them up.