Have you tried reloading the map after starting the server? If not I have a really simple script that does it automatically.
(Save it in the servers lua/autorun directory, doesn't matter what name it has)
hook.Add( "Think", "I_hate_flatgrass", function()
if game.GetMap() == "gm_flatgrass" then
Simply set your servers default map to gm_flatgrass (or a map you dont use, be sure to change the code to correspond to it) and where it says gm_construct change that to the default map you actually want.