It's been awhile, 20 + years, since I purchased a GPLR rifle, but my memory is that the "fixed sights" for the GPL was mounted on a high ramp. A drift adjustable, but on a ramp. I might be wrong, though. Just not a screwdriver adjustable sight.
At any rate, I replaced what I had iniitally installed as a traditional sight with a TOW firmly fixed sight.