There is a statement on the instruction sheet saying not to mount it on the gas block. That is the only place available on this rifle, so that's where I put it. The salesman that sold it to me never mentioned it, and the disclaimer on the instruction sheet is only readable after you remove the instruction sheet from the package. As soon as I read it I sent an E-mail to Magpul asking them about it...haven't received a response as of today.
I shot about 120 rounds, not in real quick succession, and the gas block did get pretty hot, but the sight seems to have done fine. The mounting screw stayed tight. If Magpul says that it won't hold up under the heat, then I will switch it to a metal front sight.