Your purchased items will be available for download in GOG GALAXY or at your "My account" page, as soon as you complete the checkout process and your payment is successfully completed (except where you get free content). Of course, the preorder will be delivered after the game's full release date.
You can download the purchased content from "My account" page by downloading standalone files, or install them directly through the optional GOG GALAXY app.
If you purchased an item as a gift (using "Gift this order" option) you or the gift recipient have to redeem the code before downloading any gifted items.
You can download content as standalone files (this way you can backup them directly on your computer or external storage media). To do this, go to your library ("My Account" page) on the GOG website, and click the purchased item to show the download options. You can also do this in the optional GOG GALAXY app, by selecting the game and clicking EXTRAS.
Installers for Windows games
Download the .exe file, along with all the .bin files (if the game has multiple installer files) under "game installers". Once the download is complete, double-click the .exe file and follow the instructions presented by the installer. Once the installation is complete, shortcuts will be added to your Start Menu, desktop and Games Explorer, so you can launch your game.
Installers for Mac OS (where available)
Download the .pkg file under "game installers". Once the download is complete, double-click the .pkg file and follow the instructions presented by the installer. Once the installation is complete you can launch the game using .app bundle that is present in location selected during installation.
Installers for Linux (where available)
Download the .sh file under "game installers". Once the download is complete, mark the installer file as executable. Then, run it by double clicking the .sh file or using the command in terminal and follow the installer steps to complete the installation. Once the installation is complete, you can run the game using the shortcut created in your applications menu, on your desktop, or just by launching the starting script in the installation folder.
Make sure that the "MOVIES" tab is selected before you start searching for the movie in your library. Download the .mp4 file and, once the download is complete, double-click it - it will start playing through your operating system’s default video player.
Additional content ("Goodies")
Download the .zip files and, once the downloads are complete, extract them using your system's default archive software. Once extracted, you can enjoy the bonus content within. Note that bonus content may be grouped into sections in the downloads list, if your purchased game had more goodies bundled with it (through an upgrade, or being a superior edition).
Installation through GOG GALAXY
The GOG GALAXY app must be installed in order to use this option. You can download its installer at https://www.gog.com/galaxy, or by clicking "TRY GOG GALAXY" button, found above the standalone download options.
Once GOG GALAXY is installed, and you are logged in with your account, access "Owned Games" in the sidebar, select your game and click the INSTALL button. The game's files will be downloaded and installed automatically. Once this is done, click the PLAY button in order to launch the game.
If you purchase in-Game content, the content will be available for you in the game after purchase. You can redownload your items as many times as you like, but you’re not allowed to share your GOG account with other users, as only you are entitled to download items from your account.