If anything, I'd say that Half-Life 2 ridicules the idea of the "great man". For instance, the City 17 Uprising was only possible because of 20 years of the resistance building up under the assumption that Gordon was already dead, and the uprising was only triggered by the destruction of Nova Prospekt — which was completely accidental and not even Gordon's fault. And for the first week of the City 17 Uprising, again, Gordon was completely absent and presumed dead, and yet the resistance just kept growing. All of the Combine's focus on hunting down "Anticitizen One" ended up being pointless, because the resistance would just persist without him anyways, as it always had!

So, Gordon's shoes could be filled by anybody at any point... But that's exactly what you're doing when you play a video game with a silent protagonist, anyways! Any "greatness" that Gordon has is your own. You are the "right man in the wrong place" — "being the hero" was your choice by continuing to play the game. All that the Gman really is is the sort of "middle man" between the real world and the game world — that's my understanding, at least. The Gman is such an enigmatic figure that to me the best interpretations of him are always meta in some way, rather than just taking everything at face value.

Gordon achieved this sort of messianic status because the Black Mesa Incident had been made into this legend to inspire hope in the resistance. But Gordon Freeman himself remains aware throughout the entire game that his success has always at every point relied on others' help, on the networks already in place, on the enemy's mistakes, on the little coincidences here and there. Gordon just cannot speak for himself — just like the actual so-called "great men" of history cannot speak against their own mythologization.

I mean, I have a lot more thoughts on this, naturally, including some that touch on aesthetics — but it's late and three probably very pretentious paragraphs is probably enough.