There is no conclusive evidence that Gestational Diabetes can produce a macrosomic (large baby over 4kg) baby. In fact there have been quite a few mums with GD on here who have only had babies of average weight. Either you are meant to have a big baby or not - GD is irrelevant to that. There is an article on here about the entrie concept of GD and large babies.

Take the estimate of the scan as a bit of fun, as it is no real indicator of babies size - there is a margin for error of 15% either way, meaning that baby can be 15% smaller or larger than the estimated size.