Gimpshop is when you use GIMP (the GNU Image Manipulation Program) to edit an image. As opposed to shooping or Photoshopping something, someone might say they "Gimpshopped" an image, meaning they used GIMP for photomanipulation.
Guy 1: Hey look what I just Gimpshopped.
Guy 2: You mean "Photoshopped"?
Guy 1: Nah, I don't use Photoshop, I use GIMP instead.