Some are old, but a Google search brings up a lot of articles where law enforcement note that it's the largest cash crop. Obviously, it's difficult to compare to legal crops, but the figures given by many show it to be far-and-away a bigger seller than tobacco. It at least puts the answer in doubt.
And you're right, I should have thought of Delaware Water Gap. But I didn't. As a result, the only Gap (other than the one between my ears) that came to mind was "Cumberland". But thank you for telling me what I should have thought. *rolls eyes*