Please try to download this game using GOG Galaxy - it should offer the most convenient experience for you.
First, download and install the latest beta version of Galaxy here.
After the installation run Galaxy and sign in with your GOG account.
Go to the Library, select your game and click Install. Follow the onscreen instructions. The game will be downloaded and added to the side-bar on the left. Click on it when the installation is finished and press the Play button.
If you do not wish to use Galaxy, you can download an offline installer instead, and just install the game on your computer.
To do so, go to your Account page, click the game, and download its installer file(s) under "game downloads".
Please note that if many installer files are listed (1 of 5, 2 of 5, etc.), you need to download all of them into the same folder, in order to install the game.