Connect a Raspberry Pi to a 300 MHz gate opener with this design from FlyingSaucrDude Productions:
Open Your Gate From Anywhere!
This little gadget allows you to connect a 300 MHz, 10 dip-switch remote gate opener to a Raspberry Pi so that you can open a gate with just a few lines of code.
It connects directly to an off-the-shelf gate opener remote (sold separately, unless you choose the “Controller + Gate Opener” option) and uses a 5V relay for triggering the gate opener remote and a voltage conversion circuit for powering the gate opener remote (replacing the remote’s battery). It requires only 5V power, ground, and a single GPIO pin from your Raspberry Pi.