Is it safe to seal a gas fireplace?
Where I live, it is mandatory that if you can smell the gas, you have to call a professional certified in detecting gas leaks in home installations, or if you can't afford that, emergency gas service. They will find the leak and fix it or, if it is not trivial, safely unplug your device so you could get it to repair shop, in a way that is up to a local code and will prevent gas leaks in the immediate future.
If you have a tiny leak, that now dissipates creating non explosive air mixture, enclosing it in a smaller place may create your own personal fuel-air bomb. Gas shouldn't be leaking. It should be burning creating more or less odorless exhaust gases. The smell is added to that gas so you can know when you have an unacceptable leak or when something is wrong with burning process. Leaving the real problem alone and just sealing it off is not a solution. It is creating a much bigger risk for later.