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.
Some people say that GH post them in their discord channel, but I didn't find any. Does anyone know the right place to find codes? and if you don't know then don't spam here. Thank You!