The Special Investigation Team (SIT) of Mumbai Police, which is probing the encounter, detained Divya Pahuja and her mother Sonia in Gurgaon earlier in the day and questioned them as suspects in the case, he said.
They were later placed under arrest, the officer said without providing further details.
Gadoli was shot dead by a team of Haryana Police inside a hotel near the Chhatrapati Shivaji International Airport in suburban Andheri on February 7.
The other accused arrested till now by the SIT are all policemen from Haryana, namely constables Vikram Singh, Jitendra Yadav and Sub Inspector Pradhyuman Yadav, who led the encounter team.
Vikram and Jitendra were nabbed from Mumbai, while Pradhyuman was apprehended from Gurgaon.
The team led by Pradhyuman claimed Gadoli, a history-sheeter, was shot in retaliation after he fired at them to avoid arrest. His relatives, however, alleged that it was a fake encounter and filed a police complaint after which the SIT was formed.