Dedicated server map downloading help

so im a noob at this, just recentley decided to set up a garrys mod server and everything works fine, except, unless im on a stock map people cant get in cause they obviously wont have it. some people have told me all i have to do is put sv_allowdownload 1 in the server.cfg but that sounds way too easy and of course it didnt work. I have also heard you cant do it if the serveris running off your computer, again it might be a simple fix but i dont really know what im doing. any help is appreciated.

Thanks,
Kyle

Wait wait, is this home hosted? Or did you buy it off a server hoster.

home hosted

You have to forward your ports, go here for help : http://www.facepunch.com/showthread.php?t=795119