I'm not too sure like I said I dont have much technical experience, I've seen a couple of posts saying that G&G use standard specs but others saying they aren't compatible with much so im coming up blank with that one. Thought about wrapping in PTFE tape like you said to pack out the thread as a possible stop gap solution.
I got the rail from fire support uk (https://www.fire-support.co.uk/product/angry-gun-l119a2-rail--925-inch-short-version) which says it comes with the standard AEG but it may be possible that the barrel nut was wrong?