In a trading statement the company said that after a first quarter which had seen net inflows of £126m, it had since seen about £1.1bn of net outflows.
Guillaume Rambourg returned to work last month after a probe which found he had breached internal rules for directing trades to brokers.
The 38-year-old, a major shareholder in Gartmore, lost his role as a fund manager after the probe, returning as an investment analyst. He had been suspended on 31 March.
"The substantial majority of the outflows occurred or were notified at the time of Guillaume's suspension," said Gartmore.
"In addition, following his reinstatement a significant proportion of notified alternative fund redemptions were rescinded."