Step 1: A Relay, a Couple of Resistors and Capacitors, and Let Siri Control Your Gates Using a Homekit Enabled Power Outlet and This Relay Adapter...
There are several homekit controlled power outlet adapters, such as iDevices or elGato. However, none of these can directly control a gate opener, as they output 120VAC. You will, however, need to be within range of your wifi network in order for the homekit power switch to receive commands.
This circuit uses any homekit 120VAC switched outlet to drive the coil of a double-pole double-throw relay, available at radio shack for around $10. We need to generate 2 control signals: one for "gate open" and one for "gate close". Also, these signals need to be pulses rather than continuous "on or off" signals.
The Viking gate controller board has pullup resistors on its control inputs, which pull these pins up to a 5V rail. These resistors are roughly 22Kohm, causing around 2.2mA to flow when grounded. To open the gates, the 'open' pin needs to be momentarily grounded, then released. Similarly for the "close" command.
We create the pulses by connecting a discharged capacitor to the commnd input pin, which then pulls the pin low but then the capacitor gets charged back up by the pullup resistor, returning to the high state again. The time constant is equal to r*c = 100uFx22K/2 = 0.1 second pulse. When the switch if toggled, the capacitor is disconnected from the pin and discharged by the 250ohm resistor, while the other double-through side of the relay works in opposite phase, sending a close pulse to the close pin. And that's it! no microprocessors or transistors needed!
I recommend installing a surveillance camera to observe the gates while operating. My gates are 100 yards down a hill, so I use a nest cam outdoor camera to do this, and an apple watch to issue the siri command "Hey Siri, open the gates". This project worked at first build with no modifications needed, it was that simple.
This system reacts very quickly to commands, and does not have any alert lights or sounds to notify someone near the gates that action will be taking place. Safety pressure release is usually built into the gate controllers though. Apologies for this post not having more details and photos.