How does the postman deliver, once my gates are in place?

If there is no other separate pedestrian entrance, then you have the option of a separate post box, (possibly built into the gate), a digital keypad on the gate pillar to allowing your tradesmen (or friends) to open the gate by keying a coded number (the code can be easily changed if security is compromised for some reason), or maybe a proximity tag reader.

When the gate is closed how do visitors let me know they are there and gain access?

You would usually have a wireless bell or two-way intercom system at the gate.