A look in the internet archive and it seems the last 32 bit portable was around the middle of 2015, Gimp 2.8.14 Get to September 2015 and still 2.8.14 but all 64 bit. At least that is what I make it.
Best I can suggest is this
https://web.archive.org/web/20150215223724/http://www.partha.com/downloads/Gimp-2.8-32bit-portable.exe 60 MB.
edit: Thought I better give it a try. Strangely does not work in a Win7 32 bit VM but does in a Win7 64 bit VM. However a look at the gimp.exe file and it is 32 bit.