Adi Shankara told Nirvana Shatakam on being questioned by Govindapada. Here is what Madhaviya Shankara Digvijaya tells:
Meeting with Govindapada and accepting Sanyasa
(89-96) Travelling a long distance, that young Sannyasin; with staff in hand and wearing new Kashaya dress, arrived at the forest hermitage of sage Govinda towards one evening, even like the sun at the western horizon. The shade of tall trees on [he river
bank and the cool breeze blowing through them assuaged his bodily exhaustion very soon, while the sight of deer skins and bark clothes on the pranches of bushes indicated to him that he had arrived at a hermitage. Desirous of being initiated into Brahman-
knowledge there, he sought the advice of the ascetic inhabitants of the place. As directed by them, he approached sage Govinda's residence, a cave with an entrance only a cubit in width. Tn the presence of the hermits, he went round the cave three times, and then
falling prostrate at its door, began to chant a hymn in praise of sage Govinda full of the spirit of devotion to the Guru. He said, ··1 worship Thee, the incarnation of the great serpent Sesha, who is the bed of Mahavishnu, an ornament of Shiva, and the one on whose head the earth with all its mountains and rivers rest. Thou art the
same sage Patanjali, a manifestation of Adisesha. who.sc thousand faces put his disciples to fright and who thereupon assumed a gentle form with a single face. Thou alone, as Patanjali. produced the great
work on Yoga, as also the exegesis on the grammatical treatise of Panini. 0 far-famed one' Thou didst attain to the highest spiritual realisation from the instruction of the great Gaudapada,.a disciple of Suka the son ofVyasa. Praying to Thee for instruction in the truth
of Brahman, I salute Thee, the repository.of all great virtues ,"
Praised thus, the sage came out of superconscicus
state and asked, ··Who are you?" In reply Sankara said the following, indicative of his great spiritual enlightenment: "Revered Sir. I am neither the earth, nor water. nor fire, nor air. nor sky. nor any of their properties. I am not the senses and the mind even. I am Siva, the divisionless essence of Consciousness." Hearing these words, pregnant with the spirit of non-dualistic consciousness, the sage was delighted beyond description and replied: "Dear boy,
through the supersensuous insight which the experience of Samadhi has given me, I see that you are the great God Siva come on earth in human fonn." Then, in observance of the customary rules for
establishing the relationship between the teacher and the disciple, the sage extended his feet through the mouth of the cave, which the would-be disciple worshipped with all devotion and ceremony.
One may have an inborn knowledge of the Truth, but it is the scriptural dictum that one should be instructed duly by a teacher. So Sankara perfonned the worship of the teacher, and through the devoted service of him, became the object of his loving affection.
Highly pleased with the disciple's service, the sage imparted to him the knowledge of Brahman through the four Maha-Vakyas (great Vedic sentences) Prajnanam Brahma (Brahman is pure Conscious-
ness): Aham Brahmiismi (I am Brahman); Tat-twam-asi (Thou art That); Ayamatmii Brahma (This Atman is Brahman). He was then taught the Vedanta Sutras of Vyasa, and through them, the innermost essence of the Vedanta philosophy. Vyasa was the son of Parasara, and Suka, of Vyasa. Suka had as his disciple
Gaudapada, and Gaudapada had as his disciple Govindapada, from whom Sankara now received initiation in the knowledge of Brahman. Govindapada was considered an incarnation of Adisesha,
described in the Puranas as the serpent supporting the worlds, and it was from him that the world teacher Sankara learned all the Sastras as a disciple, with the undertaking that he would do his best to spread that knowledge among men.
The above bolded(first bolded) is a part of Nirvana Shatakam.
Regarding Gaudapada, Sankara just got vision of him and he found him delighted as Sankara had correctly interpreted Gaudapada Karika.
The only vision of Gaudapada is described as:
The Vision Of Gaudapada
(33-53) Sometime after he was free from the disease, the Acharya, while sitting on the sandy bed of Ganga for meditation, saw before him the form of sage Gaudapada. He shone like a night-lotus blooming under the glory. of evening clouds reflecting
the light of the setting sun. In one hand of his was a Kamandalu of sparkling metal resembling the inside lustre of lotus flowers, while the finger tips of the other counted a rosary ·of Rudraksha beads, round which honey bees hummed, mistaking them to be
blue lotus buds. The Acharya prostrated himself before Gaudapada in great excitement and then stood before him in an attitude of great respect with hands held in salutation. Gaudapada now began to speak, casting all around his glances, which resembled the waves of the Milk Ocean, and the lustre of his smile that excelled
the soothing influence of moonlight. He said: "Govindacharya imparted to you the saving knowledge of the Atman, and you have well realised the eternal truth of Sat-chid-ananda, the eternal and untainted Being. You have also acquired many disciples always attending. on you-disciples who have their minds at peace and senses. under control and who are endowed with devotion, faith, renunciation and strong urge t~ have direct· experience of Brahman. You have gained mastery over the six instinctive urges of the mind .. You have gained proficiency in the six-limbed Yoga.
You have acquired the six-fold spiritual excellences. Your mind has become one with the Eternal Being. I hope all these attainments are yours already." To these enquiries of Gaudapada, Sankara answered with great emotion and humility: "Honoured Master! By your grace all those excellences you have mentioned can certainly accru~ to me. What is there unattainable for one on
whom your gracious look has fallen? A glance of yours can make a fool a scholar, a sinner a saint, a dumb man an eloquent speaker, and a lascivious person a paragon of purity. Who in the world can understand even an iota of your vast and endless greatness?
It was only to you that SUka, the great son of Vyasa, imparted the. truth of Brahman out of the fulness of his heart. The greatness of that Suka, your teacher, was such that even at birth he was endowed with the knowledge of Brahman. Immediately he was
born, he abanondedhearth and home and became an illumined wanderer, so that his father Vyasa had to go after him, calling '0 my son! my son!' And as the father Vyasa, the commentator on the Yoga aphorisms, cried out like this, he, the son Suka, replied in the form of reverberations of voices coming from the trees.
To such an extent had that great Suka attained identification, through knowledge, with all existence. You are that limitless receptacle of knowledge that was filled with spiritual inspiration by that illustrious personage. It is, indeed, the summit of good
fortune for a devotee of yours like myself that he could see you today face to face." At these words Gaudapada said: "My child, hearing about your unique excellences, I, though unrufHed always, have come in the surge of love to meet you. With great joy I heard
from Govinda that from your mind has arisen the sun of a com-mentary that has broughtthe lotus of my Karika on the Mandukya into full bloom, spreading the fragrance of its import everywhere." Thereupon, the Acharya read out with all humility his commentaries
on the Mandukya Upanishad and Gaudapada's Karika on the same. Highly pleased with the comm.entary, Gaudapada said, "0 emperor of the realm of scholarship! On hearing your very clear and correct interpretation of my Karika, my mind is bursting
with joy and I feel an irresistible prompting to give you a boon. Choose whatever you like!" Sankara replied: "When I met you, in whom the splendour of Suka resides, I have received the vision of Mahavishnu Himself. What greater boon can be sought by me
whose thought revolves always around that Supreme Being only?" Deathless and desireless, that great sage Gaudapada thereupon disappeared, blessing the Acharya, who spent much of that night narrating to his disciples all that had happened.
Thus Gaudapada is Grand-Guru of Adi-Shankara.