There is no way to convert a gas fireplace to burn wood, unless you are very lucky and the gas fireplace is an insert in a properly functioning solid fuel fireplace (in which case you basically just remove the insert, and have the fireplace inspected).

In situations like this, you have a couple options.

Install a Solid Fuel Stove

This option will require the least amount of work, though can still be quite expensive and time consuming.

You'll have to follow all local codes that deal with this type of project, but for this example we'll use National Fire Protection Association (NFPA) standards (NFPA 211 Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances).

Clearances

You'll need 36" clearance from combustible materials on all sides (top, left, right, front, and back).

Floor protection

You'll need a floor protector under the unit, that extends 18" in all directions (except up/down, that would just be silly).

Listed Stoves

The floor protection requirements will be provided by the manufacturer, for listed stoves.

Unlisted Stoves

For unlisted stoves, the floor protection requirements are determined by the length of the legs on the unit.

0" - 2"

Stove must be placed on non-combustible floor.

2" - 6"

4" hollow masonry laid to provide air circulation through the layer, covered with 24-gauge sheet metal.

6" and Greater

Closely spaced masonry not less than 2", covered with 24-gauge sheet metal.

Reducing clearances

There are ways to reduce the clearances, by installing special materials around the stove. Check your local codes for methods to reduce clearances.

Stovepipe

You'll need 18" clearance around the stove pipe, as well as a stovepipe thimble where the stovepipe passes through walls/ceilings.

Chimney

If you don't have a proper masonry chimney to attach the stovepipe to, you'll have to use a factory built metal (A.K.A. prefabricated, Class A, all-fuel) chimney. You likely won't be able to use the same pipe used by the old gas fireplace. You also don't want to connect the stovepipe to the chimney used by your other gas appliances (furnace, water heater, etc.).

Build a masonry fireplace

If you've got the cash and time, you could have a proper masonry chimney/fireplace built (or build it yourself). This option will require modifications to the building structure, lots of masonry skills, a healthy bank account, and a good bit of time.

It is very important to ensure all infiltration paths a sealed off and adjacent walls and floor are properly insulated. Even a hairline crack in the doors can cause enough infiltration to lower temperatures in the area, though no actual moving air can be detected.

Even so, the fact that outside air can freely circulate in the combustion chamber will also be a heat sink, lowering temperatures in the area. More sophisticated devices similar to this utilize automatic dampers that interlock with the fuel supply. These are located on the exhaust flue, and sometimes on the combustion air inlet as well. I don't know if retrofitting such devices is possible on typical residential fireplaces. Such a project must be done very carefully to ensure complete safety. Failure of the system can result in carbon monoxide poisoning, so multiple fail safe measures are called for. Such measures are beyond the abilities of nearly all DIY practitioners. Only fully qualified professionals should perform this sort of work.

Even more so, though flue dampers prevent cold air from freely circulating, they are typically thin sheets of metal with poorly sealed edges, so the fire box will still be fairly cold, albeit much less so.