If you want to put the front sight on the gas block, you'll need a sight designed to be placed on the gas block and not a quad rail.
They are typically metal or steel rather than polymer. That way it can stand up to the heat produced by the gas block. Usually aren't too much higher than the polymer ones that are designed for the rail.
I've seen them in local shops and online.
Usually, they will state that they are specific for being placed onto the gas block. I guess, they are usually a little taller too, due the gas block being slightly lower than your top rail.
The polymer one probably wont melt during 20 or 30 round shooting sessions, but you'll probably shoot a little high because your front sight will be just a tad lower than the rear sight.
I'm by no means an expert and am new to the AR world. But, the good folks here on this forum just assisted me with this same matter a month or so ago.