There are two issues here. The first is why you would want to download GIMP on your iPad. Mac software will not run on an iPad. Only apps purchased at the App Store will run on an iPad. The iPad has an entirely different operating system and processor to a Mac. It would be like trying to get GIMP to run on a PC, you wouldn't think about trying to run the Mac version on a PC and neither will the Mac version run on an iPad.
The second issue is that, with very few exceptions, Safari on the iPad will not download files. The exceptions are PDFs and some photos. General files, such as .doc, .ppt etc it will not download.
Tim