This is a Garry's Mod lua guide for addon authors on how to add support for nZombies gamemodes (such as the new nZombies Chronicles). It's very easy, but instead of a single comparison, you support several matches
This is the broadest and most future-proof method. It matches any gamemode whose name starts with "nzombies" including variants that don't exist yet.
local is_nzombies = string.StartsWith(string.lower(engine.ActiveGamemode()), "nzombies")
if is_nzombies then
-- your special nzombies stuff