How do you gain credibility?
Simple..tell the truth !
Tell the truth every day to everyone and try to live by one rule - If you cannot say it in front the person then you should not say it -
I often wonder when I listen to people slam others if they would say the same thing if that person was in the room. It is not tough or courageous to speak out behind someone's back it is weak.
If you want to lose credibility then speak wrongly about people when they cannot defend themselves, if you are smart and someone is talking to negatively about others understand that they will do it to you and about you.
Credibility is earned, every day. Be truthful and if you cannot say it n front of the person you are best to keep quiet. You should be able to make your point without bringing others down.