I just recently obtained a GSG. I previously wrote in another thread how I tried FrogLube and though I feel it is a good lube, I wasn't blown away by it like everyone else. My GSG got the FrogLube treatment the day it came home, and so far we're at just shy of 500 rounds in 2 sessions and I had one failure to feed (round was nose up) and one failure to battery. I used no additional lube. Great so far but I know 500 rounds is nothing in the lifetime of a gun so I'll keep shooting and see how it goes.
In my experience (lots of experience with other GSGs through my previous job), moreso than the lube, the type of ammunition dictates whether or not the GSG is reliable or a paperweight. Obviously the hot stuff (CCI Stinger, Velocitor, etc) works great but it's all about Blazer Brass for me. Inexpensive and I find it to be universally great in all handguns. I am a huge fan of Blazer Brass for non-premium .22 ammo and I have had nothing but excellent results with it.