"Every gallon of gas creates almost the same amount of water." Not sure of anyway of comparing those two things in any direct way. As You state in your subject line, it's fossil fuels burning that has added carbon dioxide and methane into the atmosphere, resulting in the overall (but not homogenous) heating of the atmosphere, resulting the increased rate of the melting of terrestrial glaciers and thermal expansion of oceanic waters resulting in overall but locally pronounced mean sea level rise. Greenhouse gas contribution comes from many sources, not just gasoline, and it is my opinion that it would be impossible to isolate that from the data. Forest fires, burning coal seams, sublimation of permafrost, cow farts... too many variables. Would make a good talking point, but I would never trust the math.

That, and just on a gut level, the amount of sea level rise on a volumetric comparison verses that one variable would be orders of magnitude larger on the oceanic water side of the ratio.