I don't think there's enough information about Gabe Newell to make a solid determination of morality. His company makes a good product - one that focuses on maximizing the quality of the user experience instead of extracting every cent it can. That's rare and admirable, but Valve also ushered in the era of loot crates with TF2 and CS and designed some of the most predatory forms of that model.

Regarding Gabe himself, if he makes large donations to charity he doesn't advertise that fact. But he's a private person, so it's possible he's a major philanthropist but keeps it anonymous. He seemingly lacks the obvious vices of other billionaires (he doesn't interfere in politics or commit sex crimes) and hasn't been caught in any scandals. The worst thing he's done that I'm aware of is that time Valve gave a fake job offer to the Half-Life 2 leaker in hopes of getting him into the country so the feds could grab him.

He also doesn't seem to do much with his money. His main hobby is collecting knives, something he could have done with a fraction of a fraction of a percent of his wealth. He does own a fleet of yachts, but he demands they be as environmentally friendly as possible and leases them out for ecological research when he's not using them.

I'd personally rate him as neutral. He's not doing anything to make the world worse, but he's not really doing much to improve it either. The bar is just so low for billionaires that he seems amazing in comparison.