Williams, who used to be AAA-rated by Citywire, runs the firm's open ended UK and Irish Smaller Companies fund alongside the Gartmore Fledgling and Growth Opportunities, Irish Growth investment trusts, will leave Gartmore at the end of September.
Adam McConkey, who joined the firm in 2000, will become head of the firm's UK Smaller Companies team.
In this role he will co-manage UK and Irish Smaller Companies fund alongside Rob Giles.
The duo will also continue to manage the Gartmore AlphaGen Volantis hedge fund and Dublin-domiciled Gartmore UK Small Cap Best Ideas fund.
The boards of the Gartmore Fledgling, Gartmore Growth Opportunities and Gartmore Irish Growth trusts will make separate announcements in due course.
The departure of Williams marks the end of a 17-year career at the firm. He joined the group back in March 1993, which he described as a very fortuitous time when profiled by Citywire May 2007, as it coincided with a major shift in the markets after a prolonged period of recession.
Williams' investment strategy flourished when markets were awash with liquidity at the start of this century. However, as credit conditions tightened his strategy foundered.
According to Lipper, in the five years to end of August Williams posted a return of -1.1%, significantly below the 36.4% gain in his Hoare Govett Small Cap benchmark over the period. He has not been able to recover his rating since he lost it back in October 2005.
Williams has fared better in the closed ended world.
Over the last five years the net asset value on the Fledgling trust has risen 19.9% versus a 23.3% gain in the index and he managed to keep losses down to 17.3% on Irish Growth versus the 80% slump on the MSCI Ireland.
Meanwhile his Gartmore Opportunities trust, which has a much wider remit, has been the jewel in his portfolio, returning 47.4% versus a 12.7% decline in the FTSE Small Cap index.
Commenting on the departure, Gartmore chief executive investment officer Dominic Rossi said: 'It is testament to the strength in depth in the Gartmore UK Smaller Companies team that we can call upon a manager of Adam’s calibre.
'We are confident that, with the support of the hugely experienced Rob Giles and the rest of the team, Gartmore will continue to be a leading light in the specialist management of UK Small Cap equities. We would like to thank Gervais for his contribution to Gartmore over many years.'