I'm not sure how many other users outside of ourselves are actually looking to target the pc gamer niche. I've only done business with them as a customer. As a customer I can say that their service is solid.
Valve has approx 70% of the market share for PC title sales, expect your earnings to plummet during steam sales in the spring, summer, and winter. On the plus side you can do the newegg affiliate program as well and not worry about cannibalizing either stream because G2A has the better prices on PC titles.