Gas fireplace to wood burning
There's two factors here: 1) Can he fireplace physically handle a wood fire and 2) Does local code allow for it.

Regarding #1, if it's a gas insert...meaning it used to be a wood fireplace and then a gas fireplace was inserted into it, it may physically be able to convert back. If you have a proper masonry chimney and firebox, then it may very well have been originally a wood fireplace. To find out for sure, you need to contact your local fireplace contractor. 

Regarding #2, that might be the bigger hurdle--especially if you are in a metropolitan area. Lots of metro areas no longer allow you to 'revert' back to wood for air quality reasons. You may be able to use a high efficiency wood insert, thought. To find out for sure, you need to contact your local building department.