I have Flash2Pass wired into my headlamp relay on both of my motorcycles. A simple quick on/off with the high beams as you come up the driveway activates a transponder wired into the manual button inside the garage. It has worked flawlessly for years. Both bikes share the same responder code so you could do this for multiple cars as they also sell transmitter units separate from receivers.
The installation takes minutes, with a simple in-line connector that pierces the the high beam wire. The in-garage connection is as simple with a patch into the supply line for the button. Should be easy to weather proof the outside unit and adapt for your gate...
2011 A8 NWB
Phantom Black / Nougat Interior
Wood Interior Trim-Dark Walnut
Alcantara Package and Black Headliner
Driver Assist Package
Cold Weather Package
Dual Pane Security Glass
2011 TT Silver Prestige Pkg
1974 BMW 2002tii Polaris Silver (aka "The Pocket Rocket")
2007 HD Fatboy and 2010 HD Road Glide, Stage IV Rebuild and Cams (121HP, 118 TQ)