"I think wherewithal from governmental stimulus ultimately is really the cause," Jeffrey Gundlach, a billionaire bond-fund manager, said during an appearance on Fox Business.

Congress passed and then-President Donald Trump signed late last year a fresh CCP virus relief package that included $600 checks to most Americans. The checks began being sent out on Dec. 29.

GameStop stock began surging last month, spurred by retail investors urging each other to buy on a Reddit subforum. Gundlach asserted the rise was due mainly to the relief package.

"In this case, thanks to primarily, I would say, governmental policy, there's wherewithal among investors, if you want to call them that, or speculators, with government money being sprayed all over the place, with checks to people, that they have the wherewithal to put together into a real capital base. And in this case, there were 2.1 million people that somehow got organized on Reddit and managed to get about $20 billion of buying power," he said.

Large gains came, in part, because of short positions taken by hedge funds essentially betting on the downfall of GameStop, a video game seller.

A stimulus check issued by the IRS to help combat the adverse economic effects of the CCP virus outbreak, is seen in San Antonio, Texas, on April 23, 2020. (Eric Gay/AP Photo File)

Some posts on the subforum, WallStreetBets, support Gundlach's position. Users wrote that they used the stimulus money to invest. "A lot of us turned that 1,200 into way more than 1,200," one wrote this week.

But others said they invested with funds unrelated to the stimulus.

Jeffrey Sherman, deputy chief investment officer at Gundlach's DoubleLine Funds, meanwhile, told CNBC that the retail investors could see big losses if momentum changes on GameStop, AMC, and other stocks they've been buying.