One of the girls in one of my belly groups had a diagnoses that was undone after a few weeks of monitoring. I think from memory the insulin resistance peaks around 34 weeks, but if you aren't having any high results by 30ish then it would be unlikely that you have it.

It is worth fighting for the diagnosis to be reversed if you aren't having any further issues apart from the (slightly) high test. It is well known that test gives a lot of false positives. Not sure if you are wanting to VBAC, but it is going to be a lot harder if you have GD on your file.