Not sure what code you're under but the IRC allows the shutoff for a gas fireplace to be remote from the unit if:
-it serves no other appliances
-is properly labeled
-is "readily accessible"
Now, the code also requires a sediment trap to be placed downstream of the shutoff "as close to the inlet of the appliance as practical". Since it is not practical in many cases to put a sediment trap next to the appliance, most AHJs will allow it to be installed in the basement or adjoining space close to the appliance as reasonably practical but still "accessible".