Optisulin is marketed by American Remedies and not Dr. Reddy's Lab. It is a fixed dose combination of some vitamins, zinc, chromium etc. It has nothing to do with gestational diabetes.
No medication is required during pregnancy to prevent the development of diabetes even when one of the parents is diabetic. It is dangerous to give unnecessary drugs during pregnancy.
However, your wife definitely requires folic acid and perhaps iron. You can give her Globac Softules that contains folic acid and iron - one softule a day till labour.