A Gartmore statement released on Friday evening said negotiations were taking place on the basis of an offer from Henderson that was at a "slight discount" to the firm’s closing share price on Thursday 16 December 2010.
Gartmore’s closing share price on Thursday stood at 98.75p, though the shares rose by 6% on Friday to close at 104.8p amid speculation of an approach by Henderson.
"At this stage there can be no certainty that these discussions will result in a formal offer being made for the company either on these or on any other terms," Gartmore cautioned in the statement, adding that a further announcement would be made "in due course".
The firm announced in November it was seeking a buyer following the retirement announcement of star fund manager Roger Guy.
For Henderson, the deal would represent the second time in as many years that it has purchased a beleaguered rival, following the April 2009 completion of the £115m deal to purchase New Star Asset Management.