Most gas fireplaces come with specific instructions for the clearance to combustible materials. In particular, many come with a diagram showing acceptable relationships between how far out a mantle extends and how far above the fireplace that mantle must be.

Look for those clearances and read them until you understand them clearly. Then make sure they are followed when installing the fireplace.

I agree with seattlepioneer. We had a gas insert installed recently and made sure the clearances specified by the manufacturer were observed.