I don't know if the G&G uses different threads than others, but I put one of these on my (no-name) spectre, and it fits / works just awesome. I see it's out of stock right now but you could call and see if he's getting them back in:
http://airsoftdepot.ca/catalog/produ...oducts_id=1738
Or, if you can afford to wait a few weeks for shipping, this one's not as "deep" so I prefer the airsoftdepot one, but it fits my spectre as well:
http://www.aliexpress.com/item/Tacti...991843238.html