What does GD mean in netball?

GD stands for Goal Defence, So that means that you are defending the ball from going in the net.

GD always marks GA who is a shooter, this means that Goal Defence will have to try her best to intercept the pass to GA. It is very important that GD intercepts this pass as if she doesn't then GA has the chance to shoot. Although you have to intercept the pass and throw to WD (wing defence) they can then carry on the pass up to your teams shooting third.