B’sha’ah tovah!
In general, healthy pregnant women are required to fast on Yom Kippur. However, your gestational diabetes may make it dangerous for you to fast.
In this type of situation, it is best to discuss with your doctor if you can attempt the fast, if it is sufficient for you to eat and drink in shiurim from the start of the fast, or if you need to eat and drink normally. (See our article on fasting on Yom Kippur for more details about shiurim.) Then you should get back to us or a local halachic authority with that information.
Please feel free to get back to us with any further questions.
Gmar chatimah tovah!