Well, it's a Mario game. 64, specifically. Nnng. That game is quite good, but every dev seems to say the same thing these days - or is that just me being a misery?
Fortunately, at least if you're a PC gaming site, Valve boss Newell tells CVG of two other most immortally-beloved games, one of which is Doom ("made me rethink everything I thought about games - control systems, design, rendering") and the other one of which is simply enormously charming. It's a punchcard-based Star Trek game for the Burroughs mainframe, played via logic and long-winded printing.
I think he means this one, made by Don Daglow in 1971, but he might instead mean this one.
There's a browser-based recreation of what I think is the latter here. Play it, and you will be Gabe Newell. That's how it works, isn't it?