I am a linux user but have seen plenty Windows Gimp Help questions over the years. At one time the Windows GIMP help browser did not work (ok now). This in a Windows 7 64 bit (also tried a Win7 32 bit & is the same)
Before installing the help package Edit → Preferences → Interface → Help System should look like this with a notice “The user manual is not installed locally”
After installing the help package, the files go in the Gimp system in a …\help\en folder. Possible snags are Gimp is installed somewhere else or the installed version of English is not US ie. en …But worth checking that all the files are there.
Back to Edit → Preferences and there should be a “There’s a local installation of the user manual” notice and “Use interface language”.
That works here for Gimp help Browser option
That might be the problem, an up-to-date Browser (not MS Internet Explorer) & over secure. Then use the Gimp built-in option or it might be the language used, change that to US English.