Is expiating backbiting by merely praying for the one backbited and seeking forgiveness for him sufficient, instead of mentioning his virtues in the same gatherings where he was backbited?

It is not a condition for the validity of repentance from backbiting to mention the good qualities of the person backbited in gatherings, as there is no Shariah evidence for that; rather, it is an opinion held by some of the Tabi'in. Some scholars have held that seeking forgiveness is sufficient, citing the hadith (definition): "The expiation for one whom you have backbited is that you seek forgiveness for him." Mentioning the good qualities of the person backbited falls under what "should be done," meaning it is recommendable.