"Many of them have -- have said publicly how worried they are about the possibility of Mr. Trump becoming president," Gates said in an exclusive interview on “This Week” Sunday. "His unpredictability, his lack of understanding of the complexity of international affairs, his threats, his claims that he's going to make other countries do things, when, in fact, the president of the United States does not have the power to make them do things."
“So I think -- I think a lot of leaders around the world, both among our friends and potential adversaries, are quite concerned,” Gates added.
Addressing the Republican front-runner's recent foreign policy speech, Gates said it showed someone who "doesn’t understand the difference between a business negotiation and a negotiation with sovereign powers."
The former Defense Secretary also voiced concerns about Trump’s willingness to take advice.
"One of the things that worries me, Martha, is that he doesn’t appear to listen to people," Gates told ABC’s Martha Raddatz. "He believes that he has all the answers, that he is the smartest man in the room."
"And I've worked for some very different presidents – Jimmy Carter, Ronald Reagan, George W. Bush, Barack Obama," Gates said. "One of the things they all had in common was a willingness to listen to people who had experience, and then make their own independent judgments."
Gates has worked under eight different presidents on both sides of the aisle and served as Defense Secretary under both President George W. Bush’s and President Obama from 2006-2011.
His record in public service led the Washington Post's Jennifer Rubin to suggest him as a possible third party candidate, if Trump secures the Republican nomination.
The former Defense Secretary laughed off the suggestion of a possible presidential bid. "That's the silliest thing I've ever heard,” he said.
Gates also put to rest the possibility of being Donald Trump's running mate.
"One of the problems with being vice president, is if you totally disagree with the president you can’t quit," he said.
Gates made sure not to make an endorsement either, telling Raddatz, “Absolutely not.”
He has spoken favorably about Hillary Clinton in the past, whom he served alongside in Obama’s first cabinet. She is “smart, idealistic but pragmatic, tough-minded, indefatigable, funny, a very valuable colleague,” he wrote in his 2014 memoir “Duty.”
But he added one description Sunday that will not help Clinton’s standing among Democrats: hawkish.
“I think that she probably would be somewhat more hawkish than President Obama,” he said.
Clinton has been criticized heavily by her primary opponent Sen. Bernie Sanders for her vote for the Iraq War as a senator and her push for intervention in Libya as Secretary of State – although the chaos that has engulfed a post-intervention Libya is not entirely her fault, argued Gates.
“Ultimately, the failure to plan for a post-invasion or post-military operation really has to reside in the White House. It’s the White House that has to bring Defense and State, intelligence community, and others together in this planning process,” he said. “It’s not the province of just one department.”