Hi hfbamafan,
You are not the first Test Taker to face this particular challenge, and there are some opportunities that you might be able to take advantage of.
If your ADHD has been documented for several years and you've been 'treated' for it, then you might be able to get special accommodations for the GMAT. In many cases, that means more time per section and sometimes the GMAT is split into 2 parts spread over 2 consecutive days.
There is a lengthy process that you have to go through to essentially 'prove' that your situation warrants these accommodations, but if you qualify, then you might find it beneficial to have those accommodations. You'll still need to put in the necessary study time and effort, but that's true for everyone.
GMAT assassins aren't born, they're made,
Rich