The DD lite rail I put on mine fit nice.It has the same threads as the PTW and Prime WA uppers as well as G&P. The gas tube is some sort of steel ,and is a little bigger dia. than most.
The outer M4 style stock tube measures 1.120" so most stocks will slide on (some might be a bit sloppy though) my PTS CTR was a touch loose but a piece of tape inside tightened it up nicely.
Since then I have installed a Magpul PTS UBR. They said it wasn't possible
but it on and is rock solid. It's not perfect ,but until someone comes out with a UBR tube with KJ threads, this install works.
What sort of GBBR parts are you wondering about,maybe I can check if I have that part on something else here.