First things first: you need to check with your Township to see if it would be legal to add home automation to your gas fireplace. And it probably will not be. 
You should first read some of the other threads in the forum about fireplace controls. In most places in the US, it is against safety code to have any kind of remote control that can be operated out of sight, usually defined as more than about 20 feet.
And once you add smartthings control, or for that matter Alexa Control, to the ignition device, you have violated that. You may know that you never intend to be at your office 20 miles away and turn on the gas fireplace, but just the fact that you can is in itself a violation. And that can then void your homeowners insurance.
Search the forums and you will see a report every month or so of someone saying that a light came on unexpectedly or a garage door opened unexpectedly – – it does happen. To have it happen for a gas burning fireplace would be a serious hazard.
There are some fireplace units which legally qualify as in wall furnaces, but they typically cost $6000 and up and they have their own thermostat and multiple safety features built in. So essentially you are just controlling the thermostat. That’s a very different situation than the typical gas fireplace.
So again, the first step is to check whether what you are considering would be legal where you live, and second to consider whether it would be wise. Read the SmartThings product usage guidelines carefully:
If after doing those two research steps you do want to proceed, just search the forum for fireplace.