Hi Faithfully, with strict diet control, you should be fine. My SIL has gestational diabetes too and she managed to cut down her sugar intake substantially (eg no/ very very little ice cream, pastries, sweet drinks etc.), also converted to brown rice instead and low GI food, plus lotsa exercise, e.g. walks after meals so that the blood sugar level does not spike up.
When she's not pregnant, she'll go for runs too. So far, she'll only get raise blood sugar level when she is pregnant. Actually my in-laws side has very strong diabetes history too, so I'm also being careful with my diet now, not giving my baby too much sugar if possible.
For mummies with gestational diabetes, it's good to get it under control, as there is a possibility that they may be diabetic later on in life. Same goes for the kid/s, good idea to monitor their diet.
For diabetic petients, not only have to control sugar intake but also important to make sure no sugar spike during the day (eg heavy carbo and refined food can cause a spike which is not ideal), as will cause dizziness, e.g. If goes up to above 16, may need to be hospitalized and given insulin jabs.
Like Hazel said, it's best to speak with a dietitian to further understand it. With better understanding, there will be less fears and can be in control
Jia you....