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.
Best Answer
It depends on the gas insert. Many gas inserts are listed for contact so if it is your contractor would be correct. I don’t think the contractor would try and cheep out on this because a fire would be on his bond if he did not do it properly.