This is a basic template with the purpose of including all arguments / table variables to make it easily known which values can be accessed.
gameevent.Listen( "server_cvar" )
hook.Add( "server_cvar", "server_cvar_example", function( data )
local cvarname = data.cvarname // The convar namelocal cvarvalue = data.cvarvalue // The new value// Called when a convar with FCVAR_NOTIFY is changed on the server.end )