Currently the WS6318 doesn’t have built in GPS or support for an external GPS, however the new generation HL series modems, which will be pin for pin compatible with the WS6318, have the option of an embedded GPS. Have a look here for more info:
http://www.sierrawireless.com/en/Newsroom/newsreleases/2013/11-05-2013-Sierra_Wireless_delivers_new_level_of_flexibility_to_essential_wireless_connectivity_for_M2M.aspx