Buy a Centurion gate motor, as they have a solar option on the control panels, and have both the panels, the charge controllers and the larger battery typically used in these systems.
If it is an existing gate controller check the current drawn by the controller with the mains disconnected, and then the operate current of the unit. Typically you have a 30 to 100ma draw with it in standby ( depending on the remote receiver and the relay for the light being energised for 5 minutes) and 10-20A in operation. You need to have enough capacity in the battery so that you do not draw more than half the capacity if there is no sun for 3 days and you operate the gate 10 times. Typically you need more than 7.2AH, more like 16 to 20AH. Your panel should be able to charge it from flat in 8 hours, so a panel that delivers 20W will be a little undersize, but 2 in parallel will work with a simple charge controller to charge the battery and maintain it.