I think i know who murdered Gujjaral.... Abhishek (Shobha's ex-husband)
He left the show too quickly and the whole situation with Shobha finding out about him was fast-tracked. And remember back when Abhishek tried to get Karan in trouble after Shobha found out about him... he payed Rathi to say that Karan was the real drug dealer and he killed Ansh... they left that story line quickly as well.... no aftermath of that was shown... so i think Abhishek killed Gujjaral and he threatened Karan to take the blame...saying he would kill Shobha if he didn't... he did this to get revenge on Karan for losing Shobha...
This is just a guess...because i know if Gautum was the real murderer, it wouldn't be shown in such a way that the audience already knows..there would be no shock... other suspects left are Mohini.. and no one else would make sense...