fruits and vegetables are good for over all health and not just for the mind, that's pretty much common sense, however different parts of the world have certain diets and we are brought up to eat more of one thing and less of other, and its hard to modify eating habits close to the test
I went through something similar, trying to eat healthy for a healthier brain.
try eating nuts, almonds/walnuts etc.
don't forget to drink plenty of water, dehydration will impact brain function and can cause headaches.
and ofcourse exercise, light exercise yoga/walking can help immensely, I started out by just stretching before going to sleep and it turned into a full workout 4 times a week, and my verbal score improved too.
and its a good idea to avoid fatty/sugary food, takes longer to digest, and makes one sleepy.
All the best for healthy eating habit and a great gmat score