If Gore was president during 9/11, he immediately would have had 90% approval ratings, with Republicans falling over themselves to authorize whatever more-limited version of war Gore would have wanted. It's the "Rally Around the Flag" effect. When America is attacked, the President becomes bulletproof.
I'd presume Gore would have been re-elected, and with a larger margin than Bush was.
I don't agree, I think a GOP would have been much tougher on Gore (and his (D) predecessor) for failing to prevent 9/11, failing to be tough enough in the aftermath, etc. which in large part would have limited the media cheerleading we saw IRL under Bush.