What site did you try to download GIMP from?
This is the site you should be using:GIMP
Always try to download programs from the developers site.
That's the best way to ensure there will be minimal problems, if it's a safe site, which this one is.
You'll need to download two programs for Windows in order to use the GIMP: the GIMP
and the GTK
runtime environment, which is available for download on the same site. The program won't work without it. Also, if you speak a language other than English, you can select it when you install GTK.
Mirror sites, are sites other than the developer's site, where their programs are hosted for downloading.
It helps to relieve the bandwidth drain on the site, for developer's of popular programs, or sites that have limited bandwidth.