You can head over to the GOG homepage, and if you're signed in you can scroll down a bit for the giveaway banner to claim your copy of Dex. It's worth doing too, as it's a pretty great game. It has a Native Linux version too if that's important for you.
As a reminder: every hour new big deals will also appear on the GOG homepage, so you might want to check back often.
Here's a bunch of useful lists they've put up for the sale too: