Analysts projected up to £3bn of Gartmore's £20.9bn of assets could be withdrawn.
Mr Guy, who runs the firm's flagship £3.5bn European Large Cap team and is responsible for more than 30pc of the group's profits, will leave by early 2011. It was reported that he was unhappy with the handling of the internal investigation into former colleague, Guillaume Rambourg, who resigned in July.
Mr Guy, who has been at the company 17 years, also owns 5.4pc stake.
A shaken Mr Meyer said the money manager – still 24pc owned by its private equity backer Hellman & Friedman – would not settle for a fire sale of its assets amid what insiders are describing as a "corporate melt-down".
Potential buyers for the asset manager being lined up include Schroders, Henderson, Royal Bank of Canada, New York Bank of Mellon and private equity firms such as CVC.
"If we can't get the right value we will walk away," said Mr Meyer. "We need to get some stability. That could mean selling just the Hellman & Friedman stake, it could mean going to a bigger corporate company or a merger."
He added: "Getting out of the public markets is better for the firm."
But a senior source at a potential suitor said: "The problem is no one knows what you are buying.
"Half the assets could walk out the door tomorrow so it's not clear what earnings multiple you are basing any deal on. This is a particularly accident prone company. Who knows what else can go wrong?"
Mr Guy was not the only departure. Dominic Rossi, Gartmore's chief investment officer, said he was going to rival Fidelity. Darrell O'Dea, who was parachuted into Gartmore in the aftermath of the Rambourg investigation, will also be leaving.
The asset manager attempted to stem the exodus by issuing new equity grants to employees of up to 15pc of its existing share capital, as well as embarking on a £10m cost-cutting programme.
"We are a resilient group and the equity awards have gone a long way," said Mr Meyer, admitting he could not have imagined a worse outcome for the company that has been beset with problems since its flotation. "If there is something worse than what has happened to us – I can't think of it."
While the formal sales process is just beginning, Mr Meyer said he hoped to have an agreement in place before Christmas. It is understood the valuation being sought is between 7 and 9 times earnings, or between £385m and £495m, less £64m of net debt.
Gartmore was brought to market by Bank of America Merrill Lynch, Morgan Stanley and UBS.
