To relativise his wealth at least a little (not by much though, he's still part of a fucked up system): Some of it will he due to his ownership of companies with a lot of assets. In case of Valve, that is at least preferable to public ownership and the attendant pressure to make more and more profit at any cost. Worker-owned would be better, but leaving that aside:

Companies have assets. Even without their profit expectations driving (speculative) stock prices, the office equipment all has some book value, and the value of a company is comprised in part by the sum of that value. If you have a company with, say, 200k in IT hardware, and you own 75% of it, that's an asset value of 150k. Subtract, say, 25k in debts, and you're left with 175k equity, of which you own 143k.

Valve supposedly has an equity of 10 Billion USD. Gabe owning over half of it puts him at 5 Billion just for owning that company. It's not all "hoarded money", or at least not directly. Liquidating it (e.g. to donate the money) would mean selling (parts of) Valve, and I dread to think just who would have the wealth and desire to buy, and what they would do with their controlling interest.

So the wealth itself is a result of the way capitalism treats equity and ownership, so that having the sovereignty to make business decisions require him to own more than half the company. If we concede that he's less malicious than other billionaires, we'd still have to replace the underlying system, but I wouldn't immediately group him with the rest of the actively greedy and exploitative lot.

Again, it would be better to have the workers share in that wealth. That would mean giving up his control and I'm not sure he'd be willing to do that. He's not a saint. I just hope he never turns into a devil instead.

I guess. Does he not donate to any charities or anything?