Well, whether or not it is possible for Garry to update Garry's Mod to natively support Left 4 Dead content, that didn't stop perseverant modders from modifying the Left 4 Dead models so they could be used in Garry's Mod like any other model.
There are a few different model packs out there that contain different models from the game, so different packs may interest different people.
Left 4 Dead Ragdolls and Guns
Ripped from the demo by Facepunch user
MasonB964 , this pack contains primarily the ragdolls of the Infected (zombies) and the Survivors, along with the ammunition and guns from the demo.
Download links:
Survivor Ragdolls
This pack is primarily useful if all you are interested in is the Survivors, for use in comics and such.
Download link:
Hunter Playermodel
This model has recompiled for use as a playermodel by
yukoshoo, useful if you want to create a Left 4 Dead machinima with Garry's Mod, or make interesting screenshots.
However, the author has noted that this model is not faceposable or eye posable, only finger posable.