If your GX furnace seems to produce a lot of smoke around the door, or after the fire gets going and you close the door, the fire goes out, you have an airflow problem.
Obviously start by giving the furnace a good cleaning - clean the firebox, the secondary burn chamber, the vertical tubes and the horizontal tubes.
Please make sure that you never use any Creosote sticks with the catalyst disk in place. Before putting the creosote stick on the fire, remove the catalyst and keep it out for 3 days after using a creosote stick, or at least five loads of wood.
Since there is an airflow problem, please envision the path of air traveling from the blower fan through the unit and out the chimney. Somewhere in that path, there is either a blockage, or there is a failed component.
(For GX10 models, remove the blower fan/solenoid assembly by unlatching the "suitcase clip" on the right side, carefully remove the fan assembly which has the solenoid and damper plate attached. Check airflow, then replace.)
2. Second, if the solenoids are not lifting the damper plates, the air cannot get into the tubes that carry the air to the firebox. With the fan box door open (or for GX10, with the fan assembly pulled out), have someone else actuate the Fan Switch in the front control box while you watch the operation of these solenoid(s). If they are not lifting the damper plate(s), then the solenoid either has a loose wire connection, or needs to be replaced.
These are the two components that, if failed, will prevent proper operation - the fan and/or solenoid.
Now, if those components are functioning properly, then there is a blockage in airflow that must be cleaned out. Many times, GX owners think they have cleaned everything, but there is still some segment of the airflow channel that is blocked. Therefore, you must carefully check the entire path that air passes through the entire GX furnace:
3. Blower fan pushes air into the rear air box. MAKE SURE THIS GX15/30 BOX DOOR IS SEALED COMPLETELY. If the door to the rear air box is not shut tightly and sealed properly, the air from the blower fan will not go into the firebox, but rather just into the rear of the furnace. (For GX10, make sure the blower fan/solenoid damper assembly is properly seated, and the "suitcase clip" is securely closed.)
4. Next, the air has to go through those rear air tubes from the rear air box to the firebox. Those tubes can get clogged (rarely, but possible). Run your brush through those tubes to clean them out. (GX10 air flows into the firebox through the vertical channel seen from the front door in the rear left of the firebox. That channel can be removed by lifting it out of place for cleaning. The need for this is rare and only applicable for very infrequent cleaning.
5. Make sure the path of air from the rear tubes into the firebox channels is clear. Creosote can accumulate near those openings.
6. Then make sure the path is clear that air needs to pass through the firebrick into the secondary burn chamber. A brush will not fit through that channel, so you can gently use a long screwdriver, a stick or some other thin tool.
7. Make sure the secondary burn chamber is completely cleaned out using the long tool that works like a hoe. This chamber is accessed from the lower rear access door. (GX10 from lower front door.)
8. Now air passes up the vertical heat exchange tubes to the left upper rear chamber. Make sure those are clear.
9. Next air passes through the catalyst disk, if it is in place. Make sure it stays clean. Clean it only with water and make sure it is dry before putting it back into the furnace.
11. Then from the rear right upper chamber, you can see how the exhaust exits the horizontal tubes and flows into the chimney, make sure that pathway is clear.
12. Finally confirm that your chimney is clean.