Actually it's not G's that matter. Although knowing whether you are pulling G's will help you in your calcuation of how much lead to apply. What the real issue is for calculating lead is the speed of the target, and the angle of the interception between your guns and the target.
For example in Fry's water fight he is not pulling any G's, he's standing still and merely rotating as he fires his hose. This is analagous to being in an Osty or field gun. If his brother is running perpindicular to him, so that Fry has to swivel to lead him, he leads because if he fires now, by the time his water has gone 7 feet his brother has moved from point A to point B, so the water goes behind him. So Fry fires at a point ahead of his brother so that the water and his brother both arrive at Point C at the same time.
The most important factors here are: A) How far away the target is; B) How fast the target is travelling; and C) How long it takes your projectile to cover that distance. Which is why if the target is close, you lead less, far, you lead more. Target is slow, less; Target is far, more. If you have a low velocity projectile, it takes longer to cover the distance, therefore you have to add more lead than a high velocity projectile.
In order for Gs to have an effect, Fry would have to be chasing his brother, not standing still. If Fry chases him around in a circle, then Fry would experience some G's as he makes the ciricle. However the G's would have very little impact on Fry'a aim point, the most important thing is to aim at point C ahead of little brother so that the water and little brother arrive at C at the same time.
When you're in a 6 tail chase, the amount of G's is an important indicator of how much your target is changing course. The more G's, the more deviation from a straight line in front of you, therefore the more lead you apply to make sure your projectiles arrive at C the same tme he does. It's not the G's that are causing your bullets to go behind the target, it's the fact the target has moved from point A to B before they get there.
Want to prove it to yourself? Use the .target command to create a target at various distances. Go into a circle pulling G's and as the target passes by fire at various times. You'll see that even though you are pulling Gs, you don't have to lead the target to hit it, because it's always staying at a fixed point. So its not the G's you pull that determines where your aim needs to be, but whether or not and how fast and far away your target is moving.
Another example is a crossing shot. Your bogie is scissoring in front of you. You go straight and level, unload your wings and are pulling 1 G. If you fire when the bogie is in your site as he crosses, you will fire behind him. You must fire before the bogie crosses so that your bullets and his plane arrive at a point in front of you at the same time. The faster he is going the earlier you have to fire.
A good way to practice is offline against the drones circling the base. Use the Lead Computing Sight to see how much lead you have to apply when attacking at various angles.