At least on Windows gimp ,regardless of the setting used (, even if that path is unchecked in the preferences ) load always all the font in C:Windows/font
but you may add fonts to be read only by gimp locally, inside that font empty font folder, in case set the path using Edit/preferences/folder/font...once done remember to click ok before exit from preferences ( i suggest because i often forgot with very delusive results

)
That Gimp font folder may be useful also for fonts that doesn't show up...may happen that they will work if copied in that font folder
(in any case there are a few fonts that for some reason gimp can't support, but should be rare cases )