First off,grab a map off the Gmod (Garry's Mod) Workshop.This will be the map you'll be putting into your "maps" section of l4d2,which is located in Steam/steamapps/common/Left 4 Dead 2/left4dead2/maps.Once you've downloaded that,go into your GarrysMod folder.Go into its bin folder,scroll down until you find an application called "gmad".Send it to the desktop as a shortcut(this is optional).After that run Garry's Mod to install your map.Exit out after it finishes.Head into your Garry's Mod addons folder,which is located Steam/steamapps/common/GarrysMod/garrysmod/addons.Copy the .gma that is your map to your desktop.Open it with the "gmad" application.It should turn into it's "raw" files.

First off,if the maps raw files include materials,take those materials(.vmt,.vtf)and put them in your l4d2 materials section,which is located in Left 4 Dead 2/left4dead2/materials.Otherwise,download the Half Life 2 Texture Pack off of gamemaps.The mod adds all the hl2 materials and models into l4d2 and doesn't replace any of the original textures.Take the .vpk and paste it in your addons folder.

Its simple really,find your maps section of l4d2 and open it up.Take the .bsp and drop it into the maps section.To get the .bsp out,simply open up the unpacked .gma and where it says "maps" open it.You should see the .bsp.Take that out and proceed to put it into the maps section of l4d2.That's it!That's all you need know! (P.S These maps can only be loaded via the developer console,unless you've released it as a campaign,and another note,bots will not move on any gmod map unless there are navs set in place for them to)

I shouldn't really go into depth with this, but I am.You see,getting the HL2 Texture Pack adds textures and models for maps that require HL2 models.I'm certain that getting TF2 maps to work would be to download its textures and models from a website or to copy all of its textures from its folder into l4d2's materials folder,but I'm not fully certain on that aspect.CSS is different in a way,as some of l4d2's textures are actually from CSS,so copying the textures may override the textures that already exist.I saw a video that had said to copy all of CSS materials and models over to l4d2's models and materials folders.You can even put any of the maps in there as well!Only the CSS maps have navigational paths for the bots.Other than that,no other maps will have navs for them to move around on.Them being the bots.Again,try it out, show me a screenshot of a TF2 map in l4d2,it would be great to see it!