As pointed out some zwave outlets can monitor current in use, however I’m confused as to how that’s going to solve your problem.
I’m assuming the Minimote is going to be used to open/close the gate and you want a way to trigger actions (such as record video) when someone is at your gate?
If you plan on having the Minimote in your home, 400ft is really pushing the distance limits of zwave, your going to need a device (or two) in middle to repeat signals. I am assuming that the trigger circuit is not wired the 400ft into the home here.
The Minimote its self won’t open the gate, your going to need some kind of dry contact relay on the trigger circuit. The button push on Minimote would close the circuit but you would still need to add some logic to open the circuit again after a second or two.
The doorbell button idea is a good start, but I don’t think it’s going to work the way you think. First I would rather do something with a motion detector over the doorbell idea, z wave motions are not that expensive. As far as using a current sensing outlet on the power source for doorbell, probably not going to work. The doorbell will always be drawing current, in use or not. The device might draw a small amount of extra current when running, but I wouldn’t trust that as a trigger. If you really want to use the door bell as a trigger, you would need to wire in another relay on the chime circuit so that when the doorbell rings the relay triggers. Something like this should work: [RELEASE] Nexia Doorbell Sensor (DB100Z) Device Handler