Another, less concrete cost is that of incentivizing bad behavior. If I cannot sufficiently move a certain type of stolen goods, I'm less likely to attempt to steal that type of good again in the future. However, if people choose to pirate a game instead, the bad actor is not rewarded and might realize that he can no longer move that type of product.
Maybe a customer wouldn't buy the game at the full price, but since they're not pirating from the get-go, that shows they are a customer willing to spend money. Maybe they wouldn't buy the same game or as many games, but it's possible they'd still be spending some money on games through legal (but more expensive) means if they knew g2a was just piracy with more steps.
Many years ago I experienced something like that with Paypal where someone bought a license for my software and then immediately made a chargeback claim. And on Paypal unless it is a physical delivery with a tracking number, buyer always wins. I lost some time while communicating with Paypal about the chargeback.
This is where most of fraud take place and game developers lose a lot of money on it.
It damages their brand (vs. piracy, which unless you're using invasive DRM is net neutral at worst, and positive at best), generates negative sales additively (vs. net neutral to positive sales based on word of mouth advertising), and drains the development house's emotional and fiscal capital as they deal with the fallout.
I don't exactly see how this is worse than piracy. isn't it basically the same as "Customer A gets a game for free of {torrent site}. they tell all their friends about {torrent site} and their favorite bittorrent client."? the only way it's different is if most consumers have some moral objection to torrents. in my experience, very few people who work outside of software (or other IP-based industries) really care. they only pay if it's less trouble than torrents.
Maybe keys should be changed to secure hardware dongles.
But buying a G2A code is almost as easy as buying from Steam. And a lot of people who wouldn't pirate for ethical reasons don't realize it's not legit. So G2A turns loses you sales from people who would otherwise have bought the game legitimately.