According to the GC comparison PDF that has been around, the GC3 was available in brunstone/blustone, but it seems to me the GC1 and 2 came with it also. All I can tell you is I've never heard of it being an issue as a substitute for slate. Actually now that I think about it, it seems to be the only alternative I've not heard as generally considered to be inferior to actual slate, like honeycomb and all the others.
My advice would be to ask this question in the mechanics forum.