Ganga can't be the killer. if it's Mohini then she might have heard Gautam talking on the phone to someone so now she is taking the advantage of that where Gautam is not even aware that the man he hired really did killed Gujral. and don't forget that Mandira said, in order to get Karan out of jail she has to frame someone else in the house and since she knew about Gautam calling on the phone to kill Gujral since Gujral had that taped it so she is using Gautam as a mohra.
if anyone it can only be Mohini and told Karan to take the blame or she will go and announce that he is married to Taniya.