You can calculate energy density if you know your electrode radius. To calculate specific energy, use amount of catalyst (gram) per cm2.

Dear Umit,

There is no catalyst on GDL. I need to know the active surface area of GDL by itself.

If you prepare a GDL, you should be certain porosity, carbon content and basis weight of your GDL. If you dont prepare and buy from a company, physical properties of GLD is certain. For example i use GDL from mti corp. You can use the parameters of your own GDL and calculate specific capacity.

http://www.mtixtl.com/ELATGasDiffusionLayer-EQ-bcgdl-1400S-LD.aspx

well you may test Fe(II)/Fe(III) CV with different concentration of iron to calculate the surface  area of your electrode.