This depends on whether using a vented or a ventless gas log. Vented, check the chimney for pull: sit in front of fireplace and blow out a candle. If the smoke is pulled into the fireplace and flue then it's working. If a problem you'll have to check the flue installation: section seals, fire-stop, termination, etc installation. If it is working a wider diameter flue or a different termination will be necessary.
Vent free fireboxes have a heat shield; many customers ask to remove it. If the heat shield was removed (even without asking for it) replace it. There are many decorative options for the shield which is designed for exactly keeping heat off the mantle.
Finally the biggest! mistake sales people make is over-sizing the burner. In theory a 36 inch firebox will hold a 24 inch gas log burner but depending on the valve and box angle this is rarely the case.
When I was younger this is a problem I ran into several times and mis-sizing the gas log burner or not checking the "pull" of the flue was the problem every single time.
Feel free to contact us with questions or a more detailed explanation.
This is normal , I have a gas fireplace myself and has no fan built in which has black marble surrounding the hearth , The marble gets hot which transfers heat to the mantle , As long as there is no burning smell , Then it is OK , If the fireplace has a damper , Be sure it is open before lighting , My dad did this one time , Lit the fireplace and forgot to open the damper and it set off all the smoke alarms