There are core exercises you can do. They helped first time get my gap from 6 or 7 back to about 2. I am doing them again, and wearing a support band but it is harder to close the gap this time.

There is an gym that has some online vids. if you google fit2bstudio you can find it and then check out their diastasis awareness section and workouts. Even better if you can get to a women's physio that works with pregnant women who can show you how to exercise.