It is the first time the Bush administration has made such a commitment for 2009.
Gates, speaking to reporters on his way to Muscat, Oman, from a NATO summit in Bucharest, Romania, said President Bush made the pledge at the summit on Thursday.
Mr. Bush was not specific about the number of additional troops that would go to Afghanistan in 2009, Gates said. The United States now has about 31,000 troops there - the most since the war began in October 2001 - and has been pressing the allies to contribute more.
Until now, the heavy commitment of U.S. forces in Iraq has been a constraint on the ability to increase U.S. troop levels in Afghanistan. But Gates said he did not believe that would be the case in 2009.
Gates said he advised Mr. Bush to make the pledge to allied leaders in Bucharest even though the movement of the unspecified additional troops would ultimately be a decision for the next president, who will take office in January.
"The question arises, how can we say that about 2009?" Gates said. "All I would say is, I believe ... this is one area where there is very broad bipartisan support in the United States for being successful" in Afghanistan, where, by many accounts, progress against the Taliban resistance has stalled.
"I think that no matter who is elected president, they would want to be successful in Afghanistan. So I think this was a very safe thing for him to say," the Pentagon chief added.
Gates said he believed it was too early to decide how many additional combat forces the United States should plan on sending in 2009. He said it would depend on several things, including the extent of U.S. and NATO success on the battlefield this year, as well as the impact of a new senior U.S. commander taking over in coming months. Gen. David McKiernan is due to replace Gen. Dan McNeill this spring as the top overall commander in Afghanistan
McNeill has said he believes he needs another three brigades - two for combat and one for training. That translates to roughly 7,500 to 10,000 additional troops. The Bush administration has no realistic hope of getting the NATO allies to send such large numbers.
In remarks to reporters after Mr. Bush made the statement at the summit Thursday, the president's national security adviser, Stephen Hadley, said any extra U.S. combat troop deployments would be in southern Afghanistan, where fighting is heaviest.
Gates said he believed that was a logical possibility but that it was too early to say they would go to the south.
"I put this in front of the president as a possibility, as something that I thought we ought to be willing to say and do," Gates said. He added that part of his reasoning was that such a pledge by Mr. Bush would have extra effect at a summit meeting where France announced that it will send several hundred combat troops to Afghanistan this year - a decision that Mr. Bush explicitly praised.
It is widely agreed within the Bush administration and between the United States and its key allies in Afghanistan that they have too few troops on the ground to effectively fight the Taliban resistance - especially in the volatile south - and to accelerate the training of Afghan soldiers and police.
The question that has been contemplated for many months is how to find additional troops.
The administration initially pushed hard for other NATO countries to fill the gap. Having largely failed in that effort, the U.S. military now seems convinced that it will have to bear more of the load.
Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, has made clear his view that the enormous commitment of U.S. forces and resources in Iraq has made Afghanistan, by necessity an "economy-of-force campaign." In other words it has been a secondary priority amid fear of collapse in Iraq.