(Read more: Pimco CEO El-Erian resigns)
El-Erian, CEO and co-chief investment officer, will leave Pimco in March. Gross will become the lone CIO.
"I'm disappointed. We're disappointed. When he came back [to Pimco] six years ago ... we thought he would be my successor and would be in place for 10 to 15 years. That's not going to be the case," Gross said on "Street Signs."
"We're disappointed that he won't be with Pimco; that he didn't, you know, continue in the successor role for me as chief investment officer," Gross said. "But that is the way it is and we're going to move forward."
(Read more: Pimco's problem: Who can succeed the 'Bond King'?)