try white vinegar and water - which I just used to clean my wood burning xtraordinare 44. I learned about that reading posts on this forum.

For the wood burner, I dip a cloth soaked with the vinegar and water into the ashes to make a paste, I spray the glass, and the rub the cloth on the glass and it cleans off the black stains nicely. For the gas burner - you probably don't need the ash step.

There should be screws around the frame holding the glass to allow the glass to be removed and cleaned.

I found it weird that there were no instructions in any of the FPX documents (I have one FPX gas unit and one wood burner) that discussed glass cleaning - which I think is a pretty important step.