White marks on the gas fireplace glass are caused by the water vapor that natural gas produces when it burns. This water vapor contains different acids that will accumulate on the glass and will result in a permanent etching into the glass with each use of the fireplace.
The five acids in the water vapor are:
HCL - Hydrochloric Acid
H2SO4 - Sulfuric Acid
HNO3 - Nitric Acid
NaNO2 - Sodium Nitrite
HF - Hydrofluoric Acid
These chemical compounds attack the most commonly used sealant, compromising the integrity of the seal. This chemical attack, combined with the heat of the flue gas, significantly reduces the effective life span of the sealant. Water based sealants dry out and will not thermally cycle with the unit. They eventually crack, crumble and fall out of the joint.
Flue Gas Condensate components per ULC-S636-9
White glass is also an indicator that the logs may be out of position and the gas fireplace is in need of servicing.
Whenever you find any gas appliance with discoloration in it or around it, that appliance needs to be serviced!