All quartz watches have reasonable accuracy, especially when compared to their mechanical brethren (of course, discounting various highly expensive movements that seek to bridge the gap, like Spring Drive and Tourbillon). In CASIO's G-Shock world, we are promised a certain range of accuracy "out of the box" at +/- 15 seconds per month. Curiously enough, all of the CASIO watches I've bought save for just 1 (which is an SGW-300) gain some seconds per month, not lose. Some are better than others.
I have read a few testimonies on WUS stating how one watch model or another was insanely accurate, like drifting barely +0.5 sec/mo or having shown just +1.5 sec drift after 5 months of sitting in a drawer. Pretty amazing.
For me, there's one watch that did something I've never seen before. I synchronized it and then checked on it periodically, seeing barely any deviation, and then at the 1 month mark double-checked to find it DEAD-ON accurate. It's a G-2900. I've never had a G-Shock that accurate before. Even the ones I've time adjusted the best will gain or lose about 1-2 seconds per month. So, I'm of course hoping that this G-2900 may exhibit a little better thermal resistance than my other watches and keep on being accurate, but I'm not going to be upset if it doesn't. That's the innate problem with basic quartz movements--they will fluctuate in their rate of time drift. Most vintage G-Shock watches exhibit a notable gain in time per month, due to aging of the quartz crystal.
So... what's your most accurate G-Shock and how long has it been keeping great time?