At my booking appointment with DD (2 years ago), the MW told me that GD was easy to avoid by eating healthily (lots of veg to be specific). And staying away from sweets, biscuits, choccies and cake (not what you want to hear when pregnant!).
Also regular exercise would help as long as it was gentle even walking is good. Apparently exercise lowers bloodsugar and keeps it in check.
I had lots of porridge (nice if you slice some fruit into it) for breakfast.
I did ok with lunch and dinner too. Brown rice, homemade veg curries, grilled/poached fish etc etc.
But just couldn't stay away from the chocolate! I didn't get GD but I craved chocolate so so bad. And I did gain a huge amount of weight (due to choccie overload). Still trying to lose it now.
So I do recommend doing your upmost to stay away from the sweet stuff.
For what its worth, we're all different and some people will eat huge amounts of sweets and not get GD and others will eat really healthily and still get GD.
I used to see a small japanese lady who was pregnant same time as I was in my local starbucks every day. She was tiny with tiny bump and used to order 7 (yes 7) cakes and scoff them all in one go!!!
Also apparently fruit juice is bad for blood sugar spikes.