I think being over weight and having a bad diet are only increased risk factors. So just because someone has those risk factors doesn't mean they'll get GD. Conversely someone with a good diet who is not overweight can also get GD.
So i don't think you should beat yourself up over it. You didn't cause this if you have it. iykwim

GD can often be managed by diet. When you have any kind of diabetes your body is unable to break sugar down due to a lack of insulin, hence the need for insulin injections in some people. If the pancreas is producing some insulin but not quite enough, then, by avoiding sugar, often people are also able to avoid needing insulin.
If the pancreas is not producing anywhere near enough insulin (or people don't stick to their diet) then people sometimes need insulin.

Having a good diet is a great start, but sometimes even with a good diet the GD is not controlled.

Fingers crossed all is ok and you wont have to worry about needles. xx