You'll notice, after all, that most live-action stuff centered around the Half-Life universe doesn't really contain any Gordon (at least the ones I've seen)[2].
Half-Life 1 was the story of a scientist caught up in something much larger than himself. You played this hapless guy who escaped and explored worlds, with compelling confusion about who your enemies and friends were. It was dream-like and scary and mysterious and great.
Half-life 2 felt like you were being lead through a metaphorical tunnel the entire game. Literally some parts had NPCs reminding you to "go this way" for hour-long stretches. Most of the game wasn't escaping terrifying situations, instead it felt like you were merely "in transit". It's hard to have a thrilling feeling of excitement and horror as you're being nagged to get to the next plot device.
What's more, whereas in HL1 you were this scientist caught up in a mess, in HL2 you play essentially the role of a hired brute. You do zero Gordon-specific things. When you meet Barney at the start of the game, if you swapped bodies with Barney at that moment, nothing in the game would have changed at all. Congrats on that MIT degree the NPCs will remind you, now kindly plow through 6,000 bugs/zombies/cycloptic humans as we tell you what to do.
I have a inkling that the sour direction HL2 took is at least part of the reason they're having trouble with HL3 but who's to say. Most reviews say its one of the greatest games ever, so maybe I'm way off kilter.
Anyway, I think a Half-Life movie, if it covered any of the events in Half-Life 2 at all, would suffer severe problems if it tries to make a story of a mute scientist who kills his way to the 90th floor of Castle X to save the world. It's just not a very compelling story.
[1] I was lambasted for offering this opinion on Reddit. The game also has a 96 on Metacritic, with zero negative reviews: http://www.metacritic.com/game/pc/half-life-2
That makes it the #1 all-time highest scoring game on Metacritic (tied with HL1!), which seems insane to me, and several (in my opinion) better games within that genre come to mind.
[2] What I was thinking of specifically was "Escape from City 17": https://www.youtube.com/watch?v=q1UPMEmCqZo