It would be politically impossible for any US administration NOT to have responded militarily in Afghanistan. I suspect the least the American people would have stomached would be immediate massive cruise missile attacks on suspected terrorists facilities in Afghanistan, coupled with essentially the same demands on the Taliban regime that Bush initially made: "Give us Osama and kick Al Qaida out or we will come in". I suspect, however, that a "Gore Administration" would have tended to treat the 9/11 attacks more as a law enforcement issue and over time that might have weakened the response. The US might have been prepared to cut US military responses short if the Taliban regime apprehended bin Laden and offered other compromises. Of course, no invasion of Iraq. That was purely a Bush thing as far as I'm concerned.