I don't know the exact chances but I know/think it is higher if you already had GD. It is genetic/hormonal. Though there is the chance you won't have it too, my Dr. said it depends on the placenta.
I dont' know the actual stats on it but I had it with my fist baby and just went into this pregnancy assuming that I will have it again. I figured if I expect it then I won't be disappointed when I have it again...and if I don't it will be a pleasant surprise. I was tested last week (at 13 weeks) and have it again. Just be mentally prepared to do it all again and for way longer this time.
Re: GD, again