5.3 indicates an average blood glucose level of about 105 - a little higher than normal, but not diabetic. 76 for fasting blood glucose is perfect, but since gestational diabetes often resolves very quickly after delivery, it doesn't necessarily indicate anything about how things were during pregnancy.
You can have GD with a normal A1c because GD reflects the average of the past 3 months rather than current levels. Since GD generally develops later in pregnancy, an A1c test done at around the time of the glucose tolerance test might reflect lower levels from earlier in pregnancy. However, one done at the very end of pregnancy or early postpartum should, at least in theory, reflect the third trimester.
They also say it because GD that's well controlled by diet won't result in an elevated A1c, but in that case, it shouldn't be an issue for the baby, either.
I'm not a doctor. There may be something I'm overlooking (such as the effects of normal hemodilution or blood loss during birth), though I'm not seeing much about that in what I've read. It's also very possible that, regardless of the validity, a doctor is not going to take it into account. However, from what I've seeing, you have a baby with several indicators of non-diabetes-related macrosomia (boy baby, overall large and not just fat, tall family members), and an A1c that doesn't clearly indicate that you did have GD, though it doesn't necessarily prove that you didn't, either.
It's also possible that it could be a combo of mild GD and a baby who would have been large regardless.
I'm not sure if knowing whether or not you had GD really effects anything in the long run. If you had it, you would want to be aware of it for future pregnancies, and in general since you'd be at higher risk for type II DM in the future. But that could happen anyways, so it's good to be aware of regardless. As far as the baby goes, feeding a healthy diet and otherwise encouraging good health in childhood is the best thing you can do regardless.
In any case, even if you did have GD, you didn't ruin him for life. Even if he ended up with associated health problems, which isn't a given, he's not ruined.
My brother was nearly 10lbs, by the way. He's in his 30s now, and 6'4", with no significant health or weight problems.
Anyways, I'm sorry if my suggestion was overall not particularly helpful, since it can't provide a definitive answer, but hopefully it helps give you a little peace of mind.
