It’s a pretty copy paste description. Don’t know what a nylon metal is? It’s not a metal gearbox as OP pointed out, OP also didn’t get any iron sights as it’s listed.
It sounds more like a faulty unit rather than false advertisement on fps thou as AK commented too. I’d say OP should of hit up the store about it’s poor performance ootb but if it was from GBU good luck with that.
It’s the “fire breaker” description that did it for me as a copy paste effort 
It only has to have a pot metal outer barrel for it to get the “nylon and metal” description these days. 
But it’s hardened Nylon handguard What did they hammer forge some Nylon or what didn’t swing hard enough
Anyway hope the OP can get it sorted.
I had to remove a heap of material to get LDT ATM box into a classic army xm177.
First time comment in this forum. This might be the EBG gearbox. Separated gearbox have better polish than the one come with HK 416 blaster ( what your own maybe). I think you can try to hone the lower receiver just like the guy at this video. He got a separate gearbox and fit it in m4 Sijun receiver. For the gel, try ultra hard gel ( the blur one) ?. Hope it helps, this is the name of youtube video:
EBG gearbox, mẫu gear hoàn toàn mới cho súng đạn thạch đến từ hãng ATM