I do believe that Dean is correct on this point. If Al Gore had not turned on gun owners, he would have won his home state, and probably a couple of other states as well. It would be a mistake to focus on this too much, I think, though. The 2000 election was so close, particularly but not only in Florida, that you could probably fill a page with "if onlys".
If there was a single issue that made a difference in the electoral count it was the gun issue. Bush dismisses it at his peril, particularly if he's running against someone who can claim a pro-gun record.