Hello, I'm not 100% sure if ADD/ADHD/Etc. can be classified as a disability, but I'm pretty sure it is a learning disability, and (someone correct me if I'm wrong), is part of the Americans with Disabilities Act.

Anyways, here's what it says on the GMAT Information Bulletin.

"Requesting Test
Accommodations
GMAC is committed to providing access to the
GMAT exam by offering reasonable accommodations
in accordance with the United States
Americans with Disabilities Act, as amended,
and other applicable law. Testing accommodations
are available for test takers who meet
certain criteria and properly document their
request. To register as a test taker with a disability,
obtain a copy of the Supplement for Test Takers
with Disabilities. Note: Accommodation
decisions may take three to four weeks,
and you must receive an accommodation
approval before scheduling a GMAT
appointment. Score reports contain no
indication of whether a test was taken with
accommodations.
You can download the Supplement for Test Takers
with Disabilities at mba.com/accommodations
or request a copy from GMAT Customer
Service in your region.
To request an accommodation not related to
a disability, please contact GMAT Customer
Service in your region."

For the process, please refer to this link.

https://www.mba.com/mba/thegmat/schedule ... lities.htm

You should call them to confirm your situation, and what documents/testing you'd need beforehand.

I hope this helps!