Well, you have to take him as a whole into consideration. Gordon's childhood consisted mostly of quantum physics and worshiping Einstein and Stephen Hawking. He graduated from MIT with a Ph.D. in Theoretical Physics. The dude is more than just a nerd, but a flat out genius scientist. In fact, I'm pretty sure he was the youngest scientist in all of Black Mesa. When you looked at everyone else in that compound, almost everyone was pushing 50, at least.
Even with all that gray-matter, physically, he is a force to be reckoned with. The crowbar is a good example. In the first Half-Life, while all his scientist colleagues were hiding like scared little boys, he was the guy who took matters into his own hands, brandished a freaking crowbar as a weapon, and took out his rage on those damn aliens. I doubt you could ask any of old geezers to do a single pull up, let alone everything that Gordon went through.
His stoic, silent nature is a defining characteristic of him. But, if you really want to give him a voice, I say look no further than Ross Scott:
[link]http://www.youtube.com/watch?v=7J80KD4BG7M[/link]