Please provide its FD number, if you want to know its exact age.
Gas smell is usually caused by Freon leak in your Fridge, you will need a refrigeration engineer who can check the cause of leak and then repair it.
Please try this to solve the issue:
Leave it unplugged and empty with door open for around 12-24 hours.
Then plug it back (expect some water on the floor) and wait for another 4-8 hours so it gets back to optimum temperature.
The problem should be resolved.
However, if the issue appears again then it needs re-gassing and that can be done by Refrigeration engineer.
Alternatively, you may call Siemens at 03448 928 999 to book repair.
If there is anything else I can help you with, please let me know.