I don't know who is remaking the rocker gills, but just wanted to say that my 70 Cuda has the original rocker gills and they appear to be fiberglass.
Gran coupe models were some type of pot metal, I owned a 70 gran coupe convertible and the rocker panel trim was pot metal and the gill lines ran horizontally where as the 70 Cuda the lines are vertical.
71 gran coupe also were pot metal and the lines were horizontal
I saw a rocker gill broken way back in the day, it kind of looked shredded where it broke thats why I thought they appeared to be fibreglass.
I just looked at a spare one I have and challenger6pak is correct they are plastic, I should have looked before speaking out, I'll blame it on old age!