I'll do some research into this and present the results in this thread. Here's what I know so far:
Game Resolution: 5760x1080
Game AspectRatio: 48:16
I installed gmod on my system (which was blank reinstalled a couple months ago) and only subscribed to the model and maps addons shown on the tnb site. The rest of my gmod install is 100% stock. When I first completed the install, I set up my resolution and graphics quality settings and loaded up sandbox on flatgrass to test thing out. Works 100% fine with the Hud constrained to the middle monitor and the FOV accurate for my resolution. When I loaded in HL2RP, the FOV was locked at 90.
I'll load up sandbox and see if I can find any more information for you.