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.
It's not exactly pay to win but if you spend some you'll improve yourself quickly. I played this game for months and didn't spend a single dime on it and I was totally at top 100.