Are GFB blow off valves adjustable?

Yes, all of the GFB blow off valves feature spring pre-load adjustment.

This should not be confused with the noise adjustability of the Stealth FX and Deceptor Pro, which have a second system to for this purpose. Spring pre-load is used to keep the piston shut at idle, and therefore should be adjusted to suit the idle VACUUM, not full throttle boost (it has no effect on the boost holding ability of the valve). The adjustment of the valve should be made so that the piston will shut just before idle every time.