im trying to install the witcher 2 and when i click on my account and withcer 2 theres 13 parts to download and when i try to download part 1 it says i need to locate part 11 but i tryin installing part 11 first but with no luck can anyone help me out please thanks.

I'd suggest just downloading it through GOG Galaxy. It's the easiest way. You don't even need to use Galaxy to play it once it's finished installing.

Another option is to select it on your library page. Hit the "More" button that is to the right of the "Try GOG Galaxy" button. Then select "GOG Downloader links". From there, select your system (WIndows, Mac, or Linux), and your language. This will download all 13 parts at once without having to keep a browser open. Once it's finished downloading , open the 1st part (for a windows user like me, it's in the "GOG.com downloads" folder in My Documents). It'll start the installation process, and once it's done, you're good to go.

Enjoy Geralt! :)

Just like user nmorello11 said you can use Galaxy. However if you want to use the browser, you'll need to download all parts to the same folder and then the installer will work.

Use Galaxy :)