To get this achievement you need to be level 20.
Once you reach level 20 you can call Merryweather and request a air-support unit.
Provided you have the 5K required, this will net you the 5g achievement.
There appears to be some discretion and when the ability unlocks. As I am aware you have to be level 20 before you will receive the phone call off Merryweather unlocking the air-support, however you may not receive the phone call for sometime afterwards.
I think you have to have visited both Lester and Trevor before you receive the phone call.