I've just checked my spare nut and it does look like it's about 31mm . It might be worth contacting Fire support (I'd ring them) to see if the alternative nut is smaller.
Otherwise as suggested a few wraps of PTFE tape might do the job.
As an aside , if you aren't already it's worth joining the L119 Owners club on FB . Its an extremely knowledgable group and someone may have had the same issue and know a fix.
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?
Sightren got a reaction from Nick G in Fitting an AngryGun L119A2 9.5" handguard to a G&G GC16 13.5" Wildhog