Using Environment Variables in Garry's Mod.
Download a copy of the module from the releases (or compile from source)
Move the downloaded file to <Garry's Mod Installation>/lua/bin/gm<state>_environ_<platform>.dll
State can be either cl or sv, for the client and server, respectively. Platform can be one of win32, win64, linux or linux64 for 32 bit (main branch) Windows, 64 bit (x64 branch) Windows, and 32/64 bit Linux builds respectively.
require("environ")
-- Loads _G.environ
-- NYI
NYI