yasya deve parā bhakti yathā deve tathā gurao | tasyaite kathitā hyarthaḥ prakāśante mahātmanaḥ || 19 ||
19. Any one who has parābhakti—supreme love and devotion to the Lord (Deva) and as for the Lord, likewise has the same devotion to his Guru, to such Mahātmas, great souls alone, will the truths explained herein clearly illumine themselves.
Understand, there is a beautiful Upaniṣad in our Vedic tradition where the Guru was once asked by a disciple.
A disciple asks the Guru, 'Why don't you initiate everyone into the great truth of Advaita–'You are the Ultimate?'
The Guru says, 'People won't understand.'
The disciple says, 'No, no, no.' The Guru replies, 'Alright, sit next to me.'
He then calls all the demons or asuras and gives them the Advaita Upaniṣad–'You are the Ultimate.' They immediately went back to the asura loka (demonic planet) and declared, 'We are everything! Eat, drink and be merry! And destroy everybody else!'
The Guru then calls the human beings. He gives them the Advaita Upanishad, and emits the word, 'You are the Ultimate!' They come to planet Earth and say, 'Come on, enjoy! We are the Ultimate! Lets have pleasure! Pleasure is life! Pleasing the body is the goal!'
The Guru then calls the matured beings, the great souls, Mahātmas, and utters, 'You are the Ultimate!' They sit in Samādhi and get enlightened!
Understand, sometimes the same truth, if you don't listen to it, live it with authenticity, śraddha, it can be dangerous. If you live with Authenticity, one truth is enough to completely make you enlightened. The Truth about death, it can either make you more and more dhārmic and enlightened, or it can make you more and more greedy and lusty.
You can say, 'Anyhow I am going to die. Why unnecessarily waste life? Let me meditate and get enlightened!'
Or you can also say, 'Anyhow I am going to die. Why waste life? Come on, womanize, drink, enjoy all pleasures!' Same Truth with śraddha or authenticity works in a different way; and without authenticity works in a different way.
See, without authenticity, the truth of Advaita, Advaita Satya—'You are the Ultimate! You are Śiva!'—can make you a demon. Without authenticity, the spiritual strength or the spiritual teachings given to you can make you a demon. Only with authenticity, it makes you enlightened, understand.
Who is Guru
So, the basic quality for receiving the Guru Gītā is
authenticity. Śraddha, (add on Parabhakti)