Next time someone sees said message :
Please tell me on steam, so I can update the server. Garry's mod updates forcing servers to be updated are sort of rare but it may happen again in the future. Do not unistall your game or anything similar, this message just means the server was not updated and there is nothing you can do yourself besides telling me to update the server. I may not notice myself that the server needs updating unless I see the number of players is ridiculusly low, so the sooner someone tels me the server need to be updated the better.The server you are trying to connect to is running an older version of the game
I lock this message as it is resolved.