GOG Downloader is just an .exe file that can be run from any place on your hard drive. If wish not to use it any more, simple delete the .exe file (you may also want to delete the .zip file in witch you downloaded the downloader executable). There is no dedicated uninstall process (as there is no install process).

After that you will be left with a few registry entries on your computer, but they won't affect you in any way.

Remember, that the fact that you have our downloader on your computer does not implicate that you have to use it to download games. You always have the option to download using the browser.