Guru is needed for knowledge or Gyan. This is emphasized by Krishna.
Krishna explains that the best Yagya is Gyan Yoga. In fact, the Devotion or Bhakti is also the result of Gyan Yagya. To have complete devotion, Gyan Yagya is needed.
See it brilliantly presented in Krishna serial by Ramanand Sagar.
NOTE : ALL THE VIDEOS 176 VIDEOS AND EXPLAINATION OF BHAGWAT GITA IN DETAIL CAN BE FOUND IN THE SAME CHANNEL ON CLICKING THE URL GIVEN BELOW OR GOING ON TO THE PLAYLISTS, AND CLICKING 06 BHAGWAT GITA LESSONS: