What does it mean if you have gray matter in areas where there are only supposed to be white matter?

The white matter is myelinated axons of neurons. Myelin basically insulates the nerve fibers to allow for more efficient nerve impulse transmission. Grey matter is the actual bodies of the nerve cells.

Please don't allow my speculation to scare you. Wait until you are seen by a physician in person, they will be able to give you a much better answer as to what is going on.

If there is grey matter in locations that are usually white matter, it is likely an indication of some demyelinating disease process, however, demyelinating diseases are not usually a surgical problem, so another option is perhaps a growth of some sort in your brain, since the neurologist referred you to a neurosurgeon.

I hope the above didn't scare you. Try not to worry too much until you're seen by the surgeon. Best of luck.

Tend to think you were given information that had no meaning.  Meaning you were outwitted by this remark.  They couldn't come up with a diagnosis.  Migraine cause is very hard to diagnose and so you will spend a lot of time chasing this.  You will probably never have a final answer.