Gross, the bond market's most renowned investor and long known as the "Bond King," shocked the investment world nearly a year ago when he quit Pimco for distant rival Janus Capital Group.
This is the first time that Total Return assets had less than $100 billion since January 2007, before strong risk-adjusted returns during the financial crisis attracted monstrous inflows of cash from investors seeking the relative safety of bonds. Assets were $99.86 billion in January 2007, according to Morningstar data.
Investors have withdrawn record amounts of money since April 2013 because of erratic performance exacerbated by last year's departures by Gross and Mohamed El-Erian, the former chief executive officer of Pimco and Gross' heir apparent.
The Pimco Total Return Fund had cash withdrawals of $11.6 billion in January, though figures in recent months have reflected significant stabilization. Pimco Total Return has delivered returns of 0.72 percent so far this year, surpassing 85 percent of its category peers, Morningstar data show as of Aug. 31.
"We focus on investing client money using our time-tested strategy with the knowledge that it's risk-adjusted performance over the long term that ultimately matters to investors," said Scott Mather, lead manager of Pimco Total Return.
Read MoreWhat to avoid when the Fed waits: Pimco's Kiesel
The Pimco Income Fund, overseen by Pimco Group Chief Investment Officer Dan Ivascyn who succeeded Gross, continues to see huge investor appetite, posting $9.5 billion in inflows since the start of the year, according to Pimco's website.
The fund had assets under management of nearly $50 billion as of the end of August, Pimco said.
In the multisector bond category, the Pimco Income Fund is outperforming 97 percent of its category peers, posting returns of 2.72 percent as of the end of August.
Ivascyn told Reuters on Wednesday: "Defensive positioning in corporate credit and targeted higher quality bond exposure have supported performance across both funds."
Pimco, which is a unit of Allianz, oversaw $1.52 trillion in assets under management as of June 30.