For the quickswitch bind "fortnite", e.g. "bind t fortnite", then you can quickswitch between building and your last weapon just like in Fortnite. Don't worry if Gmod says you something like "unknown command fortnite", thats completely fine.

F1 - Switch to building Walls / Weapon on Slot 1
F2 - Switch to building Floors / Weapon on Slot 2
F3 - Switch to building Stairs / Weapon on Slot 3
F4 - Switch to building Roofs / Weapon on Slot 4
Flashlight (F) - Toggle Building Type Switch (then you can switch between the different building types with your scrollwheel and number keys)
Primary Fire (Left Click) - Place Structure
Secondary Fire (Right Click) - Change Building Material
Reload (R) - Rotate the Building