Several workshops offer air-conditioner gas top-up service. Is it necessary to top up air-con gas? I thought the system is a closed loop and the gas level will not fall unless there is a leak.
You are right about refrigerant in the air-conditioner being in a closed-loop system. This incidentally applies to any air-con, even the ones in your homes.
So the air-con gas top-up service is a load of hogwash and a waste of your money. If the car's air-con is no longer blowing cool air or has begun to perform inadequately in very warm weather, a refrigerant top-up is unlikely to solve the problem.
A low refrigerant level means there is a leak in the air-conditioning system and this needs to be fixed first.
Of course, a top-up will give you some relief, but the fresh refrigerant will soon leak.
While on this topic, it is important to bear in mind that periodic maintenance of the air-con in your car (as in your home for that matter) is necessary to maintain performance.
This includes flushing and evacuating the system of the existing refrigerant, cleaning or replacing the filters and then re-charging with fresh refrigerant. For the car, such a service is not required annually, but do consider doing this for every 60,000km.
Just stay away from the air-con gas top-up providers.