He had been with Abrdn for a little over four years and was most recently chief investment officer for multi-asset investment solutions.
Gars, which was once the largest fund in the UK, is now £1.2bn in size, having been as large as £20bn in 2018.
It has underperformed relative to its own target over the past five years.
It aims to achieve a return of cash plus 5 per cent on a rolling three year basis but this target has not been achieved and it has lost 7.26 per cent over the past five years.
The fund was £15bn in November 2018 when it was announced that Forest, who previously worked at Schroders, was joining Abrdn as global head of multi-asset solutions.
Russell Barlow, global head of alternatives at Abrdn, will be leading a strategic review into its multi-asset range.
He will also take charge of the Gars strategy on an interim basis.
Forest’s departure was confirmed to clients on the same day that Abrdn appointed Peter Branner as chief investment officer.
This role had previously been abolished, before its recreation this month.
The founder of Gars, Euan Munro, quit to create a rival strategy at Aviva Investors, where he became chief executive. Another group of Gars employees quit to launch a strategy at Invesco.
Forest succeeded Guy Stern, who retired in 2018.
FTAdviser's sister publication, Asset Allocator, reported last year that Abrdn's own model portfolio range had sold the Gars strategy.
They were the last model portfolio service of any size in the UK market to own the strategy.
An Abrdn representative said: "We can confirm that Aymeric Forest has decided to leave Abrdn to pursue other opportunities in the next phase of his career. We would like to thank Aymeric for the contribution he has made to the company over the years and wish him well for the future."
The firm emphasised that Forest is not a named manager on the fund, which adopts a team-based approach.
david.thorpe@ft.com
