The threading on the G&P uppers is slightly off. It's not really a different pitch as much as its just a little taller and thread are not cut as deep. ASGI actually sells a die that will fix the issue for you with little effort but you do NOT need it.http://www.airsoftgi.com/product_info.php?products_id=4264
That or you can just buy a die locally, it's a M31.8x1.5
Anyway as stated above, you don't need it. If you take a little sand paper and clean in between the threads, or a wire brush, then apply the grease and slowly screw the barrel nut on about a quarter turn, back it off, repeat each time turning about a quarter more then the previous turn til its on.
BE CAREFUL, with little effort it works though.
It's almost like the damn paint G&P uses makes up the difference in the threads. They sure put that stuff on thick.
The above information was actually given to be by someone over at SOCOM GEAR about there DD MK18 RIS II rail system and putting it on a G&P upper. Same applies to any other rail being put on a G&P upper.
With as popular as G&P rifles have become, you would think Socom Gear, Madbull etc would also sell G&P barrel nuts. People would buy them to avoid fitting them and or buying the cutting tool.