Troy makes a rail mounted front sight and also one that's intended for a gas block as do several other of the nicer BUIS like Daniel Defense. The Magpul sights however are not intended to go an a gas block, and word is that they will in fact get damaged by the heat produced by the gas block. Generally anything marketed as "Rail Height" is intended for a railed handguard and anything intended for a railed gas block will be sold as a "Gas Block" front sight.
One additional thought, if you don't have a free float tube on your rifle then don't expect super high levels of accuracy out of anything mounted on a handguard. Though the amount of motion in a drop in handguard seems minimal, it's bough to make a big difference in the accuracy of rail mounted sights. Don't get me wrong, the accuracy is still serviceable at the sort of distances you'd actually be using a backup iron sight, but I just want to give you the heads up so you're not disappointed if you get something and don't see stellar accuracy. So if you've got a drop in non-free floated handguard I really think the MBUS at great sights since they're much less expensive and your not going to get a ton more accuracy from something like a Troy or DD anyways.
