Oh, yeah. G-Flex, not J-flex... I don't know my J's from my G's - no wonder my wife was complaining... :-(
I am not sure about the heat-resistance of G-Flex. I do know that Acraglas was engineered to be a bedding compound in rifle-stocks and rifle barrels do get hot and it is made to withstand that.
I had looked around a bit and acra glass was the most heat resistant one of the readily available ones I found at the time, but I forgot the exact numbers. I think acra glass was closer to 300F while the others were not higher than 250F.