GGO the £51m investment trust managed by Mr Williams before he quit Gartmore earlier this month, said that it planned to combine with Artemis Alpha Trust.
The board of GGO had previously issued Gartmore with a notice of termination after Mr Williams?? departure.
The board of GGO conducted a so-called ??beauty parade? of 14 investment managers after Mr Williams?? resignation, before settling on Artemis.
Gartmore Irish Growth, a second investment trust formerly managed by Mr Williams, is also considering issuing a notice of termination to Gartmore, though it said this month that no action was imminent.
The loss of Gartmore Growth Opportunities deals a further blow to Gartmore, which has seen its share price almost halve since listing on the London Stock Exchange in December last year.
Mr Williams, a veteran fund manager of shares in smaller companies, worked at Gartmore for 17 years and had an intimate connection with two of the investment trusts he managed at Gartmore, having also founded the Gartmore Irish Growth trust in the 1990s.
Prior to his departure, Gartmore suffered the suspension and subsequent resignation of star hedge fund manager Guillaume Rambourg in July after he breached the company??s internal trading rules.