The CEO of DoubleLine, which manages $84 billion for clients, told CNBC he's apolitical but said, "I think it's important for investors to deal with reality."
Another reality will likely be that Trump will have a very large deficit while in the Oval Office, Gundlach added.
"He's very comfortable with debt. We know that about Donald Trump," he said in an interview with "Closing Bell " from the Sohn Investment Conference Wednesday.
In fact, he said the presumptive Republican presidential nominee is just like another man many in the GOP idolize: former President Ronald Reagan.
"Reagan was a debt-based economic guy and I think Trump will be," Gundlach noted.
"It will probably look like it's working at first. The question is, will the boost to the economy from infrastructure projects and the like off-set the potential drag from shrinking global trade."
Trump won a decisive victory in the Indiana primary Tuesday. That led both Texas Sen. Ted Cruz and Ohio Gov. John Kasich to suspend their campaigns.
So how should an investor position their portfolio? Look at arms manufacturers, said Gundlach. He would avoid companies that are susceptible to global trade slowdowns, particularly those related to Mexico and China.
While Trump may moderate some of his positions, he will attempt some of what he's promised, Gundlach believes.
"Donald will have to at least try to build a wall, at least try to do something with jobs, try to help the median America. How far he'll go, I don't know," he said. "He can't say 'I was just kidding.'"
— CNBC's Jeff Cox contributed to this report.