Along with Joint Chiefs of Staff Chairman General Peter Pace, Mr Gates is expected to face tough questioning on the planned troop deployment.
Mr Bush plans to send at least 20,000 more troops, saying it will help bring security to Baghdad's streets.
But Democrats and some Republicans have spoken out strongly against the move.
Mr Bush and his top officials have been working to convince sceptical lawmakers of the merits of the new strategy.
'Sound plan'
Mr Gates, who on Thursday appeared before the House Armed Services Committee, is now facing the Senate Armed Services Committee.
| The new strategy is not going to yield immediate results... over time, we can expect to see positive results President George W Bush |
In his opening statements, he acknowledged widespread concern over the strategy.
But he emphasised that US military commanders on the ground in Iraq backed the president's plan.
"They believe it is a sound plan that can work - if the Iraqi government follows through on its commitments," he said.
The BBC's Justin Webb in Washington says that Mr Gates and Gen Pace will be pressed hard on the wisdom of sending extra troops to Baghdad.
Gen Pace is likely to be asked why he appears to have changed his mind about a policy he did not back in the past, our correspondent says.
'Positive results'
Mr Bush made his announcement of a "surge" force of troops in a televised speech on Wednesday.
| I think this speech given last night by this president represents the most dangerous foreign policy blunder in this country since Vietnam if it's carried out Chuck Hagel Republican senator |
He said the new troops would help secure neighbourhoods in the capital from insurgents, and that this time troop levels would be sufficient to hold areas that had been cleared.
On Thursday, he said the plan was "our best chance for success", but he warned that it could take a while to see progress.
"Over time, we can expect to see positive results," he said.
Alongside criticism from Democrats - who now control both houses of Congress - several Republicans have publicly expressed their disappointment with the move, which runs counter to recommendations by the bipartisan Iraq Study Group.
Republican Senator Chuck Hagel said Mr Bush had made a mistake.
"I think this speech given last night by this president represents the most dangerous foreign policy blunder in this country since Vietnam if it's carried out," he said.
The Democrats have promised a non-binding vote in both houses of Congress on the strategy.