I don't think it is possible to exactly calculate Glass areas. The main reason is that Doors and Windows are GDL-based element types so the glass area would be returned as a library part parameter. While it is possible to schedule Library part parameters, they are not available for Properties.
So, in such I case, I would try to use an approximation value. For example, you could define a Property called "Door/Window Glazing Ratio", which you could assign a value of something like 0.8 or 0.85, or similar, and you would use an expression to calculate the Glass area by multiplying the sum of the width and height of the Door/Window with the Door/Window Glazing Ratio. That would give you a value that you can work with.
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