Pentagon spokesman Geoff Morrell said the visit would help the White House as it prepares its year-end review of its Afghanistan strategy, The New York Times reported.
Gates is scheduled to talk with Army Gen. David Petraeus, the top U.S. and NATO commander in Afghanistan, and meet with Afghan President Hamid Karzai.
The White House review is not expected to result in any major policy changes, officials said. President Obama has said he is committed to begin withdrawing the nearly 100,000 U.S. troops from Afghanistan in July 2011, ground conditions permitting.
Gates's arrival coincides with a visit by British Prime Minister David Cameron. Britain has about 9,500 troops in Afghanistan.
The trip to the Asian country came a day after Gates said he was "not particularly optimistic" that the U.S. Congress soon would repeal the "don't ask, don't tell" policy banning gays and lesbians from openly serving in the military, CNN reported.
"Before a change in the law, we would have to certify that we've made enough preparations that it wouldn't affect unit cohesion, morale, retention and recruiting, and so on," Gates said Monday. "The legislation would give us (a) great deal of flexibility. I am not particularly optimistic, though, that it will get done. We'll see."