GIMP Notes

Open C:\Program files\GIMP 2\share\gimp\2.0\themes\whatever\
edit the gtkrc file
Locate line gtk-font-name, uncomment that line
change the font from “Sans” to something else like Roboto
reload gimp
note that the font change should work as a system font update as long as font is spelled correctly and is installed.
Special note, Adobe software for the UI lately is using Adobe Clean as the typeface font
The default 'Sans' font is ugly with bad kerning in gimp.
Other options, Noto Sans, Open Sans, Myriad Pro, Roboto, Droid fonts and look at using Windows UI fonts as well to test.