I’m not entirely sure what you mean with the IR remotes… which products are you looking at?
I think you should only need one IR channel, and the remote sends encoded messages which you decode with the arduino.
For a scale model, a stepper motor or servo will give you fine control over the movement.
I have no idea what is necessary to motorize a full-size gate. I think you would want to buy an off-the-shelf motorized gate system and then interface your remote system with it.