It is important to know and have faith in your Guru, you must have check his background and to find out more information about him before you took refuge and be his student. Everyone should do that. And if you claimed yourself to be his student, one should never have any doubt in your Guru, as he is the one who will teach, groom you to be a better person and ultimately leading you to enlightenment. Of course i am saying this based on the fact that your Guru is genuine and know what he is doing and teaching. We must always check ourselves as well on why sometimes we are not progressing in Dharma and do not blame others but always self-check ourselves first.
We should always practises Guru Devotion and do not move easily by those who have lots of negativity and bad mouthing others regularly. If we do encountered these kind of situation, simply walk away or even take this as a challenge to check how strong we are towards the real practise or even our devotion to our beloved Guru.