The amount is a third more than initial projections. The Bush administration had already asked congress to approve $147bn for the war effort in the coming fiscal year.
Gates said it was now seeking another $42bn more, bringing the total war funding request for fiscal 2008, beginning on October 1, to $189bn.
Hardware over training
A sizeable chunk of the latest request will go to hardware - $11bn for new mine resistant personnel carriers alone – in contrast to only $1bn to be spent on training and equipping Iraqi security forces.
The Bush administration has repeatedly said it intends to draw down US troops from Iraq as Iraqi forces become ready to take over responsibility for security in the country.
In asking for the money, Gates said he was aware of the controversy surrounding the unpopular war.
Since September 2001, congress has appropriated $602bn for the wars in Iraq and Afghanistan, according to the Congressional Budget Office.
"I know that Iraq and other difficult choices America faces in the war on terror will continue to be a source of friction within the congress, between the congress and the president, and in the wider public debate," Gates said.
But he said US troops had done far more than had been asked of them, and "like all of you, I always keep our troops - their safety and their mission - foremost in my mind every day".
Gates also said he had ordered an investigation into whether the Pentagon could properly police the thousands of private contractors in Iraq after the security firm Blackwater USA was accused of killing 11 civilians in Baghdad.