If its a map from the Steam Workshop, then you can add a force download to the server, which will force the player to download the map to join the game, which will resolve your problem.
Name the lua script "forcedownload.lua"
Paste in this code below.
resource.AddWorkshop( "ID" )
resource.AddWorkshop( "ID" )
Replace the "ID" with the map's ID from the workshop. If you want to add more just keep pasting in IDs from the workshop, this will work for any addon from the workshop. Place the lua folder into lua/autorun/server/.
If you made the map, then you would either have to put it on a website like dropbox for FastDL, or just upload it to the workshop yourself.
FastDL:
https://developer.valvesoftware.com/wiki/Sv_downloadurlUpload to Workshop:
http://wiki.garrysmod.com/page/Workshop_Addon_Creation