Bond fund manager Jeffrey Gundlach has long been warning and preparing clients for what could be a Donald Trump presidency.

Gundlach believes a Trump presidency could actually come with a bump in GDP through increased government spending. At a conference in May, he noted that Trump is actually "extremely comfortable with debt."

He also acknowledges that confirmation that Trump will be president may initially be unsettling to everyone including players in the markets. However, that may actually be a good thing for investors.

"[W]hen Trump is at inauguration ... and you’re scared to death … that’s probably a buying opportunity," Gundlach said during a webcast on Tuesday.

That's right. When everyone's panicking, the smart money will be buying.

It's time-tested investment advice

It's a counterintuitive way to think about the scenario, but it's also how veteran investors think.

"One of the lessons I learned over the past 24 years as a research analyst (first job was Kidder Peabody), is the best investment decisions are 'uncomfortable,'" FundStrat's Tom Lee said in a note to clients earlier this month.

"Be greedy when others are fearful, and be fearful when others are greedy," Warren Buffett says.

Buffett has been preaching this kind of thinking for a while. In fact, he wrote about it after one of the worst stock-market sell-offs in history.

“I can’t predict the short-term movements of the stock market,” Buffett said in a NY Times op-ed dated October 16, 2008. “I haven’t the faintest idea as to whether stocks will be higher or lower a month — or a year — from now. What is likely, however, is that the market will move higher, perhaps substantially so, well before either sentiment or the economy turns up. So if you wait for the robins, spring will be over.”

“In the 20th century, the United States endured two world wars and other traumatic and expensive military conflicts; the Depression; a dozen or so recessions and financial panics; oil shocks; a flu epidemic; and the resignation of a disgraced president," he added. "Yet the Dow rose from 66 to 11,497.”

On Friday, the Dow (^DJI) closed at 17,675.

Buffett has previously noted that his company and the US economy will be fine no matter who becomes president.

When it comes to risk in the financial markets, it's always something. There are always reasons why not to buy and sometimes there are even reasons to sell. But time and time again, the most frightening times end up being the most lucrative buying opportunities.

"Things are going to get pretty scary," Gundlach said.

For opportunists with time, that could be a good thing.