Can I use a fireplace that is setup for gas to burn wood?
Burning wood in a gas fireplace constitutes a serious life-safety hazard.

In General

Using a fuel other than that for which a fireplace is designed may ignite a structural fire in the surrounding construction and/or discharge toxic gasses into habitable space.

Discussion

Gas fireplaces are fuel burning appliances. Each fireplace is constructed to handle the heat and other products of combustion produced by specific fuels. Each fireplace is designed to distribute heat and discharge the products of combustion safely both in the moment and continuously over the life of the building.

A gas fireplace operates within well defined parameters and is an engineered system. It is not designed to handle the byproducts of a wood fire, it is not designed to properly draft a wood fire, and is not designed to operate over the range of possible temperatures that a wood fire may produce.

Sure, Anything is Possible...But

If sufficient resources are devoted to conversion, it may be possible to 'replace' the gas fireplace with a wood burning one. However, doing so is likely to entail significant demolition, construction, time and energy.

Creating a safe wood burning fireplace will always require experienced expertise and substantial craftsmanship.