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.
Currently GH doesn't allow SG purchase in my country so it would make sense to be able to buy on the app(Thnx to google) But i haven't seen any buy option on mob. :/