But he said that the United States should know pretty soon whether Iraqis were living up to their part of the deal and increasing their own forces.
Gates spoke as the Bush administration worked to persuade a skeptical Democratic-led Congress to accept Bush's troop buildup as the last best chance for reversing Iraq's slide.
"All Americans know that the stakes in Iraq are enormous, and we all share the belief that the situation is currently unacceptable. On this we are united," Secretary of State Condoleezza Rice told a White House briefing.
Bush's new strategy, announced Wednesday in a prime-time address to the nation, increases U.S. forces in Iraq by 21,500 and demands greater cooperation from the Iraqi government.
Asked how long that buildup might last, Gates told the briefing, "It's viewed as a temporary surge, but I think no one has a really clear idea of how long that might be."
Democrats voiced deep skepticism over the buildup.
"We're not going to baby sit a civil war," Sen. Barack Obama, D-Ill., told NBC's "Today" Show.
He said the Democratic-controlled Congress would not undercut troops already in Iraq but would explore ways to restrict the president from expanding the mission.
Sen. Dick Durbin, D-Ill., told CBS' "The Early Show" that since the new Democratic-led Congress convened last week, "questions are now being asked of this administration that haven't been asked for almost four years."
Democratic options were limited, however. Party leaders have mulled a resolution of disapproval, but that would be nonbinding, and there also has been talk of attaching a host of conditions to approval of a spending bill to cover the costs of the buildup.
Gates told reporters that he is recommending an overall increase in the military of 92,000 soldiers and Marines over the next five years, bringing the overall total to 202,000 in Marines and 547,000 in the Army worldwide. Bush said last month that he would propose extra troops for the armed forces, which have been strained by the protracted wars in Afghanistan and Iraq.
Gates also said that to ease the strain on U.S. forces in Iraq, he would have to cycle some reserve units back to the war zone faster than current Pentagon policy, which is to mobilize those units for a year after at least five years of being inactive.
Gates said today's "global demands" made that change necessary, but said it would "allow us to move closer to removing the stress on the total force."
Asked if the new U.S. and Iraqi offensive would go after Muqtada al-Sadr, the anti-U.S. radical Shiite cleric, Gates said, "All lawbreakers are susceptible to being detained or taken care of in this campaign."
Sadr is a key ally of Iraqi Prime Minister Nouri al-Maliki.
In remarks prepared for delivery to the Senate Foreign Relations Committee, Rice stressed Iraqi obligations for troops, money and the political will to allow the Bush plan to succeed. She promised oversight, without giving specifics.
"Iraqis are in the lead; we are supporting them," Rice said. "Improvement in the security situation, especially in Baghdad, will open a window of opportunity for the Iraqi government to accelerate the process of national reconciliation. We can and will measure whether this work is being done."
Meanwhile, a coalition of labor, anti-war groups and liberal organizations was announcing a multimillion-dollar advertising and grass-roots campaign against the commitment of extra troops.
A new AP-Ipsos poll found approval for Bush's handling of Iraq hovering near a record low - 29 percent of Americans approve and 68 percent disapprove.
In his 20-minute speech, Bush took responsibility for mistakes in Iraq and outlined a strategy he said would pull it out of its spiral of violence. The plan would increase the U.S. troop presence from the current 132,000 to 153,500 at a cost of $5.6 billion.
"If we increase our support at this crucial moment, and help the Iraqis break the current cycle of violence, we can hasten the day our troops begin coming home," Bush said.
In remarks prepared for delivery at Thursday's House Armed Services Committee hearing, Gates offered assurances that the military command stands behind the president. "Your senior professional military officers in Iraq and in Washington believe in the efficacy of the strategy outlined by the president last night," he said.
Members of the panel voiced skepticism ahead of his appearance. Chairman Ike Skelton, D-Mo. called Bush's plan "three and a half years late and several hundred thousand troops short."
House Republican Leader John Boehner, R-Ohio, chided Democrats. "If Democrat leaders don't support the president's plan," he said, "it's their responsibility to put forward a plan of their own for achieving victory."
While Republican House and Senate leaders stood with Bush, other Republican lawmakers bluntly rejected the president's strategy. Among them were Sens. Olympia Snowe and Susan Collins of Maine, Sam Brownback of Kansas, Norm Coleman of Minnesota and Chuck Hagel of Nebraska.
"This is a dangerously wrong-headed strategy that will drive America deeper into an unwinnable swamp at a great cost," said Hagel, a Vietnam War veteran.