Mattis was reprimanded by the Marine Corps for telling a San Diego, California conference in 2005 that "it's fun to shoot some people".
"I'll be right up front with you, I like brawling," he had said during a panel discussion.
"You go into Afghanistan, you got guys who slap women around for five years because they didn't wear a veil," Mattis said. "You know, guys like that ain't got no manhood left anyway. So it's a hell of a lot of fun to shoot them."
On Thursday, Gates dismissed concerns about the comments, saying appropriate action had been taken at the time - Mattis was reprimanded and told to choose his words more carefully – and the four-star general had learnt his lesson.
"I think that the subsequent five years have demonstrated that the lesson was learnt, Gates said.
Al Jazeera's Patty Culhane reports on the controversial choice for Centcom head
Retired Brigadier-General Mark Kimmitt, who has held a number of posts in the US government and military, echoed Gates' remarks.
Pointing out that Mattis' one-off comment was made five years ago, he told Al Jazeera that the general had since proven himself as a statesman, including by working with 27 nations in a Nato command position.
From 2007 to 2009, Mattis served as Nato's supreme allied commander for transformation as he also led the US Joint Forces Command which "focuses on supporting current operations while shaping US forces for the future", according to its website.
Mattis had "the right experience for the job" and was the "right man for the job", Kimmitt said.
Mattis, who had been due to retire, was picked for the Centcom post after a shakeup following inappropriate comments made by General Stanley McChrystal, which led to his June 23 sacking as the US and Nato commander in Afghanistan.
General David Petraeus vacated his post as Centcom chief and agreed to assume command of the Afghan war after McChrystal and his aides were quoted making dismissive remarks about senior Obama administration officials in a Rolling Stone magazine article.
"Obviously in the wake of the Rolling Stone interview, we discussed this kind of thing," Gates said on Thursday. "And I have every confidence that General Mattis will respond to questions and speak publicly about the matters for which he is responsible in an entirely appropriate way."