I'm not sure what you are asking. Your screenshot is always saved in a .jpg format. You can open up your GIMP, then "open" a file from browsing your own folders until you find your screenshot. It will open it just fine on GIMP; no uploading needed.
BUT ... if you edit/manipulate your file in anyway, it will save it as a .xcf file (GIMP file), and just FYI, that is a monstrous size of file. You cannot upload it here (or probably in other websites as well), as that type of format. You need to "save as" your file in a .jpg format. And remember to bring down the quality right before you flatten/export it (it is defaulted as 85%, but you really don't need any larger than 50% or even less than that), to make the file size a lot smaller (quality won't change that much). There ... you should end up with a .jpg file and it's back to the format of your original screenshot again.