"My brother bought this and I ended up building him a gun using this. Externally, the body is extremely high quality. Everything fits tightly together, except for the rear body pin. However, this was expected, because I have seen G&P guns before and have been impressed with their overall quality.
Internally, the quality is just as high, but there were a few problems for me. Worst of all, the bushings were plastic. I ended up having to use some old G&G brass bushings that make the gun sound like crap. So if you plan on buying this, if it ever comes back in stock, buy 8mm bearings! The gearbox was 8mm, not 7. Everything else was very high quality, and does not need replacement. The gears are still working after ~1000 rounds. I also replaced the cylinder with a systema energy cylinder because the stock one was unported.
All in all, you should shim, grease, and adjust the AOE of the gearbox because it needed it. Replacing the bushings is necessary if you want the gun to last. Also, i suggest replacing the cylinder if your inner barrel is any less than 509mm (m16 length). Otherwise, I think this was money well spent, and an amazing deal as well.