- Create an account @ GOG
- Click 'Go to Giveaway'
- Scroll down to Serious Sam and click
- Add to Library
- Wait a couple of minutes
- Go to your game library and either;
- download with GOG Launcher or download the game files only
Unsure if this process is in the correct order. It's there abouts.
"GOG.com sells games that are completely DRM-free. This means that there is nothing preventing or limiting you from installing and playing the game. As such, games from GOG never come with Steam keys."
Thanks to HotUKDeals