How can I access a resource site outside my gate not in the open word it says send a transport but..

To get resources from the sites around your city you need to use the Explore option. This unlocks at level 12 and you can use the construction vehicles to send out. You have to send them for at least 4 hours at a time. Check https://www.appgamer.com/last-shelter-survival/strategy-guide/what-is-exploring-and-what-do-you-get for more details