Zombie Panic! Source is a standalone cooperative, survival-horror, first-person-shooter! Set in the middle of a zombie apocalypse, players start as a small group of survivors attempting to stay alive. Each map will have its own objectives to complete and win the round or survive by a period of time.
The upcoming update (3.0.3) will have full Workshop support, through and through. This means the server will be able to send its installed addons to the players that are joining for the first time. This will force the players to install the required addons so they are able to play on the server (if it requires X addon, or has Y maps from the Workshop).
Workshop Downloader will update, download, or verify if the user has the required addons installed. This pop-up will only occur when the player is joining the server.
Note: Due to the fact that we don't have access to the Source engine's code, we cannot remove the "missing map" dialogue (if the map is being played).