Easter eggs are often so complex and so map-dependant, you won't get around them without scripting. Easter eggs have steps that have you do puzzles or objectives that fit the map, such as overwriting control panels, opening secret rooms, etc.

To make such an easter egg, you will need to make a Map Script. It requires you know how to code Lua and gmod. Using that, you can control all elements in the map, any gamemode features, player teleports, doors, anything at all (all the gamemode does is code too). I can't really tell you how to code the easter egg since it's gonna be so different, but you can see what I have done with Imprisoned here:
https://github.com/Zet0rz/nzombies/blob/master-workshop/lua/nzmapscripts/nz_zs_obj_station%3BImprisoned.lua

You can end the game by calling the function "nzRound:Win()" with the message you want it to display.

wow ok, thats something. Ok, maybe i might need to learn how to code and alot of coding stuff.

ya, was just going to ask a question about this, I am just getting into lua script and this is going to be HUGE help

You can use certain maps such as the Inside Chernobyl map where you have to blow up the reactor which will unlock a door.