What weight BB's are you using and have you tried setting the hop for that weight? I and most of my team use 0.25s in AEGs, as we find they give a better, more accurate flight over 0.2s, but you do need to make sure you have sufficient hop set. Find yourself somewhere with at least 50 yards of range, set up a target and try to hit it without changing the sights. If the BB is falling short, add a little more hop (small changes). Fire off another couple of rounds. Are you getting nearer? If so, try a little more hop. Any improvement or does the BB decide it thinks it's a rocket and heads for the sky? If it does, back off to where you were, as you are probably at about the max range for your setup. If it still falls short, add a bit more hop until it does go skyward, then again back off so that you aim to get the maximum range without the BB going up. Once you've reached that point adjust the sights so that they match the point where the BB hits.
If you find the BB starts to turn left or right, you will probably find that the hop rubber isn't sitting exactly central on the barrel, or the barrel isn't quite central in the hop unit. You'll need to take it out of the upper receiver and check it by eye, holding it with the hop unit as near to vertical as you can, then look down the barrel from the front and slowly add hop, watching what happens to the bulge you can see in the rubber. If it's in correctly then you should see a smooth even increase in the bulge. If it looks at all uneven or out of line then you may need to replace the hop rubber or more likely the hop bucking, the small piece of rubber that sits on top of the bulge and under the adjustment arm. Replacing it with an H bucking usually helps with consistency. Also the way to check accuracy is to aim at a target about 40 yards away and carefully (if you can) watch the flight of each BB fired on semi. Do they all go roughly in the same direction or does every shot go in a different direction? If they are all in roughly the same direction then the gun is firing fairly consistently. If they're all different then you probably need a new hop rubber & nub.
As regards your gearbox, G&G boxes are usually fairly well set up in the factory, so unless you are experienced at taking one apart, leave it as it is for now, as it can fly apart under spring pressure if you don't take care and putting it all back without knowing where it all came from can lead to problems.
As regards barrel length, is yours one with a short length hand-guard, with the front sight only an inch or two from the end of the flash-hider (probably 229mm barrel) or one with a 7 inch hand-guard and about 4-6 inches from the front sight to the end (probably around 363mm). If so, those tend to shoot around the 280 fps (229mm) or 315fps (363mm) range as they use identical gearboxes. So 50 meters is about the limit of range for them anyway. That's why I use my G&G CM16 (229mm barrel) purely for CQB games, as it lacks a bit of range in woodland, although team mates who have the slightly longer barrel get on reasonably well in woodland. For anything longer you probably need to change to a TM hop unit and rubber, where fps isn't a real factor (most TM guns shoot around mid to high 200s and still have great range).
Either way, the G&G is a good starter rifle but for the price you are never going to get it firing as well as the better, more expensive guns without spending a bit of money changing the barrel, hop, rubber, nub, etc, in which case you might be better selling it and going for a more upmarket one like the G&G Top Tech, ICS, G&P, etc.
