Play games
Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
A few days ago, when I decided to get Stellaris via GH, as the price in SG was very much worth it and I was planning to get the game anyways, I ordered it. And when the key arrived a few days later, I found out that the key didn't just give me the base game, but it gave me the Galaxy Edition, which contains a few extra DLCs compared to the standard edition. I can't say whether if you will get Stellaris whether you will also get the Galaxy Edition, but even if you won't I would highly recommend you to go for this reward!