The whole propose of a gasket is to deform to seal off the imperfections in the flange. A metal gasket would be stronger than the flange face of a grp flange. Using a non full face metal gasket risks cracking the flange.
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Yeah, I've never met the FRP flange that could take the preload necessary to activate a spiral wound. Remember that rubber effects a seal at 0 psi (not in practice, but still), spiral wounds need upwards of 10,000 psi.