Why do I say this? Two reasons.
Firstly, it was Clinton's (and following him Gore's) intention to respond to the October 2000 attack on the USS Cole by sending special forces into Afghanistan to destroy the Al Qaida camps. The proposed raid had been put on hold until US intelligence confirmed that Al Qaida was indeed behind the Cole attack - confirmation that only came in January, at the time of the transition to the Bush administration. Following the principle of disavowing anything that Clinton was for, the Bush team dropped the ball, and there was no response to the Cole attack.
If in a Gore administration the Afghan camps had been destroyed, it's possible that Al Qaida would have been weakened enough to be unable to mount the 9/11 attacks. And would also have seen the consequences of attacking the US directly. As it was, the lack of any response to the Cole attack could only have emboldened Al Qaida, while leaving their structure intact.
So it's possible that 9/11 would have been averted right at the beginning of a Gore administration. But it's also possible that the planning for 9/11 would have continued undeterred in Hamburg.
So let's assume that the 9/11 attack preparations continued. As we later learned, there were a lot of disparate pieces of information available that put together would have been enough for the Gore administration to round up the plotters. Would that have happened?
I had the opportunity a few years ago to talk to a counterterrorism advisor (now with the Sate Department) who had worked in the Clinton White House. He told me that yes, in a Gore administration, 9/11 would probably have been averted, using the precedent of the 1999 millennium bomb threat to explain why.
When in 1999 rumors of an attack on the US scheduled for Jan 1 2000 surfaced, the Clinton White House set up a dedicated situation room to deal with the possible attack, and daily sent out requests to federal agencies for any scrap of information that might be useful, as well as a reminder to be on alert. And an alert immigration officer did in fact stop the would-be LAX bomber.* So there was a precedent for what a President Gore** would have done in the months before September 2001, and if all the pieces of information out there (e.g. people taking airliner flight training who didn't seem interested in taking off or landing) had been collated in a Gore White House attack threat situation room - well, as I say, it's probable that 9/11 would have been averted.
And in his book Against All Enemies, Richard Clarke said the same.
*An unanswered, and unanswerable, question: would the immigration officer have let the bomber pass though without the White House promptings to be on heightened alert?
**The counterterrorism advisor told me that in the Clinton White House, it was Gore who was the one more concerned by, and focussed on, the possibility of terror attacks.