You have to click on the banner near the top of Gog's Homepage to receive the game. The freebie is only available until Sunday.

There are also up to 7 games available via Gog connect (if you own them on Steam but not Gog): https://www.gog.com/connect

Additionally, if you make any purchase , you get Fantasy general as a bonus, and Everspace if you spend more than $15 USD.

If you want a purchasing suggestion today, try Tower of Time (70% off, $6.59 USD): https://www.gog.com/game/tower_of_time . The game is only 6 months old and has Very +ive reviews on Steam (where it is only 40% off). It is also one of the few games now to have lower than US regional pricing (~12% lower).