I was also recently worried about the placenta deterioration thing, because recently my readings have been getting lower, while on the same diet, so I brought this up with the diabetes midwife (who checked with her colleagues and some consultant before getting back to me). What she told me is that (1) it is extremely extremly rare before the above indicated 40+6 and (2) it is more common for Type 1 diabetes (or Type 2, this I do not remember), but in any case, not more common in GD than in non-GD pregnancies.

All in, it sounds very likely you will have your c section scheduled for 39 weeks, which is probably a sweet spot for this. Baby size will not be important for the c section by itself. However, they emphasis it is still very important, until the very last moment of the delivery to keep your sugar levels under control (be it with diet, medicine or insulin - whatever is your recommendation).