How dangerous is gestational diabetes?

The main short term danger of any type of diabetes is Hypoglycemia, or low blood sugar. A blood sugar level that gets too low can cause rapid breathing, pale/clammy skin, sweating, dizziness, headache, lower blood pressure, altered mental status, hunger, loss of consiousness, and if untreated beyond that it can cause death.
Initial signs of low blood sugar can present quickly when sugar levels drop, but recovery is rapid also with administration of glucose. If there are any signs of low blood sugar, drink a glass of juice or regular soda, then eat a meal. That should bring your blood sugar levels back to a manageable level. If those steps do not help, seek medical care urgently.
There are also dangers if your blood sugar gets and remains too high (Hyperglycemia). These effects take longer to appear but can be suggested by: warm, dry skin; intense thirst; vomiting is common;rapid, deep respirations; low blood pressure; rapid pulse; a feeling of restlesness merging into a comatose state if left untreated. Hyperglycemia responds to treatment 6-12 hours after treatment begins.
Your OB will be able to advise you better on treatment options, and it is very important you follow his or her advise. I hope I helped.
My source is "Emergency Care and Transportation of the Sick and Injured" 8th ed

JeffD902- EMT-B

If you really follow your diet...you will be fine...if you don't it is very dangerous for you and baby

It's controllable, but you have to follow your diet and treatment, it's crucial to do it. One major thing is that your baby can grow larger than the normal weight, he can also develop diabetes.

I had this problem with my recent pregnancy, and did a lot of information on it as well:

Gestational diabetes is generally not dangerous to the mother or the fetus as long as it is controlled. But if excessive sugar is allowed to circulate in a mothers blood, and thus to enter the fetal circulation through the placenta, potential problems for both mother and baby are serious. Women who have uncontrolled gestaional diabetes are at risk for having a baby too large, as well as for developing preeclampsi ( pregnancy induced hypertension.)

If the mother follows the doctorc instructions, she will have almost a good of a chance of a normal delivary as a non-diabetic mother.

In woman who have gestation diabetes, she should be checked a few months after delivary to be sure blood sugar levels have returned to normal.

Prevention: Good diet, weight control, and regular exercise will reduce the risk and any complications.