The installation file is a self-extractig executable. If you have GIMP installed in the default location, i.e. in the GIMP-2.0 folder under your main Program Files folder, just execute the file
INstaller can't work because that "the default location, the GIMP-2.0 folder in Program Files "does not exist:
Gimp folder for Gimp 2.6.x is called "GIMP-2.6" NOT "GIMP-2.0"
Something similar to install in 2.7, that installer will only deflate the content in a new bogus "GIMP-2.0" folder..
.only with obsolete version as 2.0 or 2.2 (not sure about 2.4) that may work