I had GD, and my daughter was (and still is!) on the small side so you won't necessarily end up with a giant baby! :)
After I was diagnosed I went to the clinic, where they explained what I needed to do to keep sugars under control. Basically, it's down to eating smaller portions of good carbs at regular intervals so you don't get blood sugar spikes. So, yes, the haribo is out for now, but the post-baby snacking to make up for it is brilliant :) I did a couple of weeks of monitoring blood sugars after meals to see what things gave me sugar spikes, and trying to adjust my diet accordingly. They also keep a closer eye on you with some more regular scanning and appointments - I switched from local midwife care to hospital consultant care, but mainly saw the diabetic specialist midwives rather than the consultant.
I used to do things like eat 2/3 of dinner, and then the remaining 1/3 a bit later before bed. I snacked on more protein rich foods like cheese/nuts/beans (am veggie, so meat was out), and I went cold turkey on the cake and chocolate. Actually, I ended up maintaining weight in the last trimester, and getting back into pre-pg clothes within 2 weeks, which was a bonus. I certainly wouldn't have done that if I'd had free reign over cake and sweets!
I ended up on insulin to control sugar levels, but actually it wasn't as bad to do as I thought. And due to that (plus a couple of other indicators) I was induced at 38 weeks. But, obviously you don't have to agree to anything you don't want.
It's a bit of a faff with some more monitoring and diet watching, but plenty of people do get GD and it normally disappears once baby is born.
Hope that helps :)