This GSM system is one way system. That's why is a no cost operation, the GSM receives a call and if that number is in the list it will open the gate, other wise it will be ignore and hung up, keep your gate safe. Two operating modes are available, - Have any telephone or mobile call the GSM mobile number and that call will trigger the controller on your gate or garage door and open it.. You will need the GSM module and an active sim card. You need to pay for the sim card to keep the phone number of the Sim card active. This is usually around $60 per year. Not all sim cards are suitable, we always use telstra as they work, and you can give it say $20 credit and after 12 months you will have to redo the sim card or the phone number will cancel.
The GSM module is small enough to go into your gate or garage door controller box, and wires up to low voltage power 12-24v terminals and two more wires usually connected to the remote exit button terminals. All on most control boards so easy to do yourself. It can be tricky getting the sim card setup with the app on your phone but once done, it is a good reliable system. You can share the sim number with family members so any large family can belong to the system. You can even set a timer, so the gate can only be opened between 9am and 5pm etc. Outside these times the normal remotes will work and so will spare remotes, wireless keypads etc.
The main thing to look out for is that GSM units are 2g, 3g, 4g, or 5g. There are a lot of cheap GSM modules on ebay selling for $50 but they are not telling you that they are selling a 2g which is gone in most places, and 3g is going off at the end of 2021 so only 4g and 5g will work next few years. Watch out for that.
If you have any questions, give me an email or market place. or look here https://gatehousesecurity.com.au/collections/gsm/products/gsm-global-system-for-mobile-communication