I have misunderstood your problem. I thought you were talking about a fake fireplace.
You have a potentially dangerous situation and it needs to be addressed. Soot is unburnt carbon particles, soot can also be a sign of carbon monoxide! Please do not use this fireplace again until we figure out what is wrong. The list below can cause what you are experiencing.
1. If this is a vented fireplace check to be sure your damper is open.
2. If so, check to make sure a squirrel or bird hasn't built a nest in the chimney or flue.
3. You could have negative pressure in the winter, (air sinking down the chimney) instead of going up the chimney. If this is the case try priming your fireplace by using a hair dryer set on hot and blow it up the chimney for a while before you lite the fireplace.
4. Your air/gas mixture is too rich, causing the flame to smoke. If this is the case you will need to have your local gas company adjust the air/gas ratio.
5. One thing is for sure, you have soot in your fireplace. That means your orifices are probably stopped up, this is compounding the problem. You will need to have the orifices blown out with compressed air.
6. Also you are not getting a clean burn from your gas, this means you are wasting gas and not getting any benefit from it.
7. Please install a smoke alarm nearby, with a carbon monoxide detector.
PLEASE, CHECK ALL THESE THINGS I HAVE MENTIONED ABOVE BEFORE YOU BURN YOUR FIREPLACE AGAIN.