I was diagnosed at 28 weeks as well, but so far am managing to stay controlled by diet (now 35 weeks)
At first I was a bit up in the air with no real info as to what was gonna happen with the birth, but I think half of that was cos THEY couldn't tell me until they had a better picture of how I'm doing! At 32 weeks I had a scan and by then had been controlling my glucose levels by diet for 4 weeks, so THEN I got answers from my ob; as long as I continued fine, and baby continued to measure fine, then there is no reason I can't have a normal pregnancy and birth. Which for me was a huge deal; it means I can stay at my local birthing centre (3 mins walk from my mums back door!) Rather than having to drive 1.5 hours to the closest hospital!!!
If I have to go on to medication/insulin then I will have to go the long drive to the hospital. I imagine if my blood sugars go really crazy, despite medication/insulin, then they'd be pushing for an induction or something sooner than if it was maintained OK, but I haven't enquired about those details!
The issue seems to be more about the control; if your blood sugars are under good control, either by diet or with the help of medication/insulin, then they let you go further. Here anyway!! If you are struggling to control your levels then it becomes safer for baby to be born a little early (38-39 weeks) than stay in there with wonky blood sugars.
I am just hoping I pop on time/a little early this time (last time I went way overdue) cos while I'm confident to stand up for myself and not be induced for sheer convenience/unfounded fears, I am not looking forward to fighting that battle with a GD pregnancy adding to the doctors harassing....! Come on baby, not long now! Lol.
There is a fantastic group on Facebook for GD mammas if you're on there; its very active and has a lot of help. Fasting numbers seem THE most common to cause issues in GD, and are almost impossible to really control by diet, so don't feel bad needing medication for them especially!!