In addition to the important issues that @Ryan780 brings up, in most places in the US it is illegal to add a gas fireplace Control of any kind, including a phone app, that would allow you to turn on the fireplace unit from more than about 20 feet away. So that includes any use of SmartThings. You might know you never intend to turn on the fireplace while you are 10 miles away at the office, but just the fact that you could is a violation. (Which can also void your homeowners insurance.)
So the first thing you need to do is check with your local township to make sure it’s legal to add your fireplace Switch to an automated system.
It may depend on the exact model. Some of the High end fireplace units count as inwall furnaces. These typically have their own thermostats and cost five to ten thousand dollars each, as well as requiring yearly inspections. If you do have one of these models, you will probably have more options under the fire safety code.
Next is the issue of system reliability. Check the forum and you will find multiple reports of lights turning on by themselves or garage doors opening unexpectedly. While these events are rare, they can happen, so if out of sight Control of your fireplace is legal for your unit, you will probably still want to add a device with a manual shutoff timer, not just a switch.
So you may be able to do it, but there is some research to do first.