You will have a higher chance of having it in your next pregnancy, but it's not a given. I had GD in my first pregnancy and at 28 weeks into my second pregnancy I'm okay so far (touch wood!).
However, because I had it before, the hospital are being extra cautious with me and I am being monitored for it and have been since I was 11 weeks. This meaning that I had to attend a diet class, have been doing the blood sugar testing 2 or 3 times a day and having fortnightly visits to the hospital to check up on me! It's all a bit overkill really as my blood sugar readings have been fine overall, especially considering I'm not really sticking to the diet like they keep telling me I should be! I've also been told that I will be having extra scans again at 28, 32 and 36 weeks - although they have now finally conceeded that if all is looking good at next week's scan then I should be able to cut down or stop the blood sugar testing!
I don't know whether this level of harrasment, err I mean looking after, is down to individual hospitals or a nationwide standard so you may not have to go through it all again next time