Most of the new enemies seem to the the ones that you saw(but never fought with) in the citadel so they really don't need much of an explanation. As for new weapons, they could just say the Combine created them due to the rising threat from Gordon and the Resistance.
A couple things I did. Sorta new to Garry's Mod, I just downloaded the new version today.
Gregory uses his hidden mental powers to smite his foes!
Alyx Vs. The Strider
I played it before. I had some lag and bug problems though so I uninstalled it. I'll probably give it another shot in the next beta version or when it comes out of beta.
I'm not bothering with any form of HDTV until it becomes standard and I can actually afford it. I am going to get a nice flatscreen TV in a few months though.