The executiveâs decision was announced by Gartmore at the same time as it revealed Goldman Sachs has been hired to undertake a strategic review of the business.
Dominic Rossi, chief investment officer at Gartmore, is also departing to join rival firm Fidelity International.
Jeffrey Meyer, chief executive at the firm, said: â2010 has been a difficult year for the company.
âThe board has therefore appointed Goldman Sachs to evaluate the strategic options available to the firm. These may include the possibility of a sale or merger.â
Mr Guy ran Gartmoreâs Â£3.5bn European Large Cap fund in collaboration with Guillaume Rambourg, another fund manager who left the firm earlier in the year.
The latter departed following an investigation by the Financial Services Authority into whether he broke internal rules.
Figures from the firm for the third quarter showed that assets under management grew by four per cent during the period.
However, Gartmore saw Â£700 million taken out of its capital base as clients withdrew their funds.
Gartmore is looking to make Â£10 million worth of savings as part of its review.
By Jim Ottewill