Two ways to adjust the net gas flow to the carrier, restriction or bleed.
Restrictive adjustment is by-far the most common.
Usually, by a setscrew that narrows down the size of the gas path.
A bleed system works by bleeding off excess gas.
Robinson M96 works this way.

JP is the most popular adjustable block for the AR.
I make a 0.625" low profile lightweight block that will fit under LaRue or DD handguards and allow an adjustment out the front.
In a 0.750" or larger gas block, there is not room to do it this way.

Someone also mentioned the MGI adjustable gas tubes.
MGI offers them in carbine or rifle length.
They basically add a small block to the gas tube and add a setscrew that will restrict gas flow.

MGI has sold me the adjuster blocks and valve screws so that I can custom install them on any length gas tube.
For instance, if you want a mid-length adjustable gas tube, you can't buy that anywhere else.
Another example is if you want the adjuster block placed pointing out through an existing hole in your handguards, or maybe to one side.
This would require the block to be custom installed to fit your specific application.

Quoted: A custom AR15 barrel would be needed ... or a modified one at least.

If it turns out to be for the 0.875" barrels that Armalite makes, there is no problem to turn down DPMS 0.940" gas blocks seats and re-profile the front end of the barrel to make the 0.875" gas block fit.