I don't think 'No. 2' is right, but he's definitely up there I'd say. Without him, the industry-shattering changes that came from Half Life 1, Steam, the Source Engine, Counter Strike, and so on wouldn't have happened. We'd be looking at a totally different gaming landscape right now that I can't even begin to speculate about. Maybe it'd be better, maybe it'd be worse. But looking at how Epic Games has been trying to develop their own Steam, compared to the insanely consumer-friendly and pioneering approach Valve has taken with Steam itself (e.g. how they're still the only 'big player' to allow free, and complete controller customisation on an in-game basis), and I'd be more inclined to say worse.
However, creatively, he hasn't really been as big of a factor into the series he reigned over than Miyamoto did. Half Life 1 and 2 were revolutionary titles but I don't think they were as much 'Gabe Newell's' as, say, Super Mario Bros. and The Legend of Zelda were Miyamoto's. They were collaborative projects within Valve instead of ideas born from a single person. Then with games like Portal, Counter Strike, and so on he was more the person that helped propel those games to success, instead of the one who created them in the first place. Though all throughout it's clear that the games Valve makes wouldn't be as technically impressive without him; he clearly knows his stuff in that regard.
Any which way I don't think the dude should be seen as any less important in the industry than your Miyamotos and Kojimas simply because he's more tech and business-focused than creation-focused. Don't think he really deserves the meme 'GabeN is God' stuff (though it's often hard to tell how much people actually mean with that), but he definitely deserves to be considered as one of the 'big names' within the industry.
I think what's interesting to take away from it though is that the 
