Glassfibre Reinforced Gypsum (GRG) Gypsum is a mineral. It will not burn. Laboratory fire tests conducted on varying thickness of G.R.G. (with 5% to 6% glass fiber content and without face coating) substantiate this non-combustibility, also show no appreciable fuel contribution and zero flame spread. The temperature of G.R.G. will not exceed 212 degrees F. until all chemically combined water is removed.