As many of you know, this map does sadly not include a Nav Mesh. You can however generate one yourself from the console in Garry's Mod.

Type the following commands in the console to generate your own Nav Mesh.

Generate Navigation Mesh: Save Navigation Mesh: Load Navigation Mesh:
Please note: nav_generate takes a while, and I mean a WHILE. That's why it's best to save your Nav Mesh so you don't have to generate a new one the next time you play.

Check the Valve Developer website for more commands in the Source Engine

Have fun :)