I'm not sure if you found your answer to this or not, but I'll try to help you if you haven't finished this project yet. Most gas fireplaces have a minimum clearance to conbustables. More than likely you have a gas fireplace that has a 1/2 to 1" clearance on the back, plus you have to consider the venting, does it vent straight out the back or does it vent verticle. If it vents out the back, you might not be able to move the f/p back any further. As for the front, as long as it's not a wood fireplace you should be fine finishing right up to the fireplace with whatever you want. Find out the make of your f/p and look for their website and they will give you an owners manual for your f/p that will give you all the f/p specs that you will need. I hope that this helps.