Its a very bad idea to put wood in a dedicated gas fireplace. if you do you will probably exceed the temperature design limits of the appliance and likely start a fire somewhere. Gas does not burn any where near as hot as wood and thus does not require the extreme safety measures needed with a wood burning fireplace. If your fireplace is a wood burning type with an added gas log then you can simply remove the gas log appliance and burn wood as normal.
Gas appliances only require a class B flue. That is a flue with an inner wall and an outer wall. This is because while it gets hot, it is not hot enough to start a fire when the exhaust pipe is encased in another layer separated by an air gap. Fireplace flues require a class A vent which is stainless and instead of two layers it has three with an air gap between all. The center is for the hot gas and the other two provide insulation to prevent it from getting too hot. Usually the gap between the two layers is used to bring combustion air in to the fire box. This makes for a more efficient fireplace. You typically find this in wood burning stoves so equipped.
Short story of you want to read:
Many years before I became an HVAC specialist I was in the home improvement business. It was the mid 1970's and I was siding a very old house in the cold of winter. It was bitter cold and the homeowner suggested we use his old wood garage as a shop. He had a wood burning pot stove that he used to heat it. The flue was a single wall pipe and it penetrated the roof through a 24" X24" flashing and a similar sized hole in the roof. It looked sufficient to discipate the heat. We stoked the stove until the flames were literally going up the flue. It got red hot but we figured that would just add to the heat in the garage where we sorely needed it to cut the vinyl siding. The cold was making it very brittle and it would crack when we cut it. We were able to get it warm enough to keep the siding workable. As I was busy cutting pieces I looked up and saw the roof was catching fire due to the heat that had transferred to the flashing. I was frantically trying to find water to extinguish it before the whole garage went up. I ran to the house and fond the hose bibs frozen. The homeowner wasn't home and I was in a panick. I frantically knocked on the door of the next door neaoghbor who shared the driveway. She was hard of hearing and didn't respond. I then made the decision to just enter and went down the basement and filled a bucket with water at the deep sink and ran back out to see if I was in time. It was still burning but had not spread so I threw the water up to the low ceiling and it went out. By this time the lady was aware something was going on and I returned to explain and asked if I might get another bucket to make sure I finished the job. She graciously agreed and I managed to avert a disaster. When the homeowner returned I explained what happened so he would know it wasn't safe. That was my best lesson in the laws of thermodynamics. Never under estimate the amount of heat that can transfer from a hot surface to a cold one.
