If you combine the output of a GSM controller with that of a set of safety photocells installed as a presence sensor (the cheap and easy option but without differentiation between vehicles and other objects like persons or animals), opening of the gate without the presence of a vehicle becomes quite improbable. An even better sensor for detecting vehicles would be a loop detector but it is more costly in terms of material and installation and may not be applicable in your case.
Optimally, the control board of your gate operator should support an 'open only' button command (to come from the combined output of the GSM controller and presence sensor) and automatic closing and have safety photocells and all other required safety features (safety edges, force limiting) installed and configured. Ideally, the board should support preflashing, so that a flashing light connected to it starts flashing a few seconds before the gate starts moving (important especially before the closing movement). You could also connect a buzzer along with the flashing light so you have both visual and audible movement warning. You can also install a free-exit sensor (in the simplest form, it could be another set of safety photocells installed as a presence sensor and connected as an open-only button command, provided no outsider can jump a fence and get on the inside). You should provide a way for a person that gets locked inside the car park to exit (an alternative exit for pedestrians or perhaps a keypad or a keyswitch) and, as additional safety, a way to contact some manager.
BTW, programmable receivers that support activation/deactivation of remotes tend to be difficult to manage (you usually have to keep a list that tells you in which memory-slot is each remote stored and then you can empty that slot thus deactivating the remote, then you have to reactivate it and assign it to the same or a different slot which you must also record etc - this is usually achieved with a very primitive interface and in practice, takes very strict bookkeeping and more than a pinch of effort). Medium or long distance RFID controllers are also an option but the cost and complexity of most systems don't make them very attractive for small installations. Bluetooth controllers usually operate either as 'virtual keypads' with a common PIN for all users (not what you want) or as a presence sensor that activates when registered cell phones bond with it (which may be what you want but typically has the same managing problems as a programmable RF receiver - for example, one of the standalone devices I know of, only supports 10 cell phones).
Optimally, the control board of your gate operator should support an 'open only' button command (to come from the combined output of the GSM controller and presence sensor) and automatic closing and have safety photocells and all other required safety features (safety edges, force limiting) installed and configured. Ideally, the board should support preflashing, so that a flashing light connected to it starts flashing a few seconds before the gate starts moving (important especially before the closing movement). You could also connect a buzzer along with the flashing light so you have both visual and audible movement warning. You can also install a free-exit sensor (in the simplest form, it could be another set of safety photocells installed as a presence sensor and connected as an open-only button command, provided no outsider can jump a fence and get on the inside). You should provide a way for a person that gets locked inside the car park to exit (an alternative exit for pedestrians or perhaps a keypad or a keyswitch) and, as additional safety, a way to contact some manager.
BTW, programmable receivers that support activation/deactivation of remotes tend to be difficult to manage (you usually have to keep a list that tells you in which memory-slot is each remote stored and then you can empty that slot thus deactivating the remote, then you have to reactivate it and assign it to the same or a different slot which you must also record etc - this is usually achieved with a very primitive interface and in practice, takes very strict bookkeeping and more than a pinch of effort). Medium or long distance RFID controllers are also an option but the cost and complexity of most systems don't make them very attractive for small installations. Bluetooth controllers usually operate either as 'virtual keypads' with a common PIN for all users (not what you want) or as a presence sensor that activates when registered cell phones bond with it (which may be what you want but typically has the same managing problems as a programmable RF receiver - for example, one of the standalone devices I know of, only supports 10 cell phones).