Gates said Trump's pronouncements on international affairs have had "some contradictions," such as imposing tariffs on Chinese imports while looking to Beijing to help rein in North Korea.
"I have no idea what his policy would be in terms of dealing with ISIS," Gates said. "I worry a little bit about his admiration for Vladimir Putin."
As for Trump's likely rival, Hillary Clinton, Gates said he believes she can handle the job of president and doesn't let politics interfere with her judgment.
"Generally speaking, she is more hawkish than President Obama," he said.