Gauranga (the golden one) refers to Lord Caitanya, who has a fair complexion. The Lord's acceptance of the role of a devotee in the eternal form of Lord Sri Caitanya Mahaprabhu is another of the Lord's wonderful features. A conditioned soul cannot reach the absolute Personality of Godhead by his imperfect endeavor, therefore Lord Sri Krsna, in the form of Lord Gauranga, has made it easy for everyone to approach Him. As Lord Gauranga is the form of a devotee, Nityananda Prabhu (a manifestation of Balarama) appears in the form of a devotee's spiritual master. Together, they are referred to as Guru and Gauranga, or Nitai-Gauranga.
Lord Caitanya and Nityananda are identified in sastra as Lord Krsna and Balarama. In the Krsna Vrindaban incarnation, the two brothers were engaged as cowherd boys and friends of the gopis, sons of mother Yasoda and Nanda Maharaja. These same two brothers again descended as Sri Krsna Caitanya-Nityananda, appearing simultaneously in Bengal in order to dissipate the darkness of the whole human society.
Nityananda Prabhu was born and raised in the village of Eka-chakra in Bengal as the son of Padmavati and Hadai Pandita. He only stayed at home until the age of 12 years, when a sannyasi came to the house of Hadai Pandita, begging to have the pandita's son as his brahmacari assistant. Hadai Pandita immediately agreed and delivered his son to him, although the separation was greatly shocking, so much so that Hadai lost his life after the separation.
Traveling with the sannyasi, Nityananda wandered to all the major holy places of pilgrimage, behaving like an avadhuta, a divine madman. During this time, he knew that Sri Caitanya Mahaprabhu was busy sporting his own childhood pastimes, and had not yet revealed his true identity. It is said that for many days He lived at Mathura with the sannyasi, and at that time He heard about Lord Caitanya Mahaprabhu's pastimes in Navadvipa. When Sri Nityananda was touring the sacred forests of Vrajha at the age of 20 years, he knew that it was now time to join the preaching mission of Lord GauraChandra, who had just recently began his ecstatic manifestations of divine love. Therefore he quickly returned to Bengal and met Sri Mahaprabhu at the house of Nandan Acharya in Navadvipa.
Understanding that Nityananda Prabhu had arrived, Lord Caitanya sent His devotees to Him, and thus there was a meeting between Sri Caitanya Mahaprabhu and Nityananda Prabhu. One day, Lord Caitanya Mahaprabhu exhibited to Lord Nityananda Prabhu a six-armed form bearing a conchshell, disc, club, lotus flower, bow and flute. Thereafter the Lord showed Him His four-armed form, standing in a three-curved posture. With two hands He played upon a flute, and in the other two He carried a conchshell and disc. Finally, the Lord showed Nityananda Prabhu His two-armed form of Krsna, the son of Maharaja Nanda, simply playing on His flute, His bluish body dressed in yellow garments. Nityananda Prabhu then arranged to offer Vyasa-puja, or worship of the spiritual master, to Lord Sri Gaurasundara. But Lord Caitanya carried the plowlike weapon called musala in the ecstasy of being Nityananda Prabhu.
Nityananda and Lord Gauranga enjoyed many preaching pastimes together. In one famous pastime, they delivered the two rascals, Jagai and Madhai. Jagai and Madhai were two brothers born in Navadvipa in a respectable brahmana family who later became addicted to all kinds of sinful activities. By the order of Lord Caitanya, both Nityananda Prabhu and Haridasa Thakura used to preach the cult of Krsna consciousness door to door. In the course of such preaching they found Jagai and Madhai, two maddened drunken brothers, who, upon seeing them, began to chase them. On the next day, Madhai struck Nityananda Prabhu on the head with a piece of earthen pot, thus drawing blood. When Sri Caitanya Mahaprabhu heard of this, He immediately came to the spot, ready to punish both brothers, but when the all-merciful Lord Gauranga saw Jagai's repentant behavior, He immediately embraced him. By seeing the Supreme Personality of Godhead face to face and embracing Him, both the sinful brothers were at once cleansed. Thus they received initiation into the chanting of the Hare Krsna maha-mantra from the Lord and were delivered.
After Sri Mahaprabhu accepted the renounced order of life, Lord Nityananda accompanied him on a trip to Jagannath Puri. On the way, Nityananda detoured the Lord first through Shantipur to see Sri Advaita Acharya. After taking sannyasa, the Lord wanted to start for Vrndavana, and for three continuous days He traveled in the Radha-desa (places where the Ganges does not flow). He was in full ecstasy over the idea of going to Vrndavana. However, Srila Nityananda diverted His path and brought Him instead to the house of Advaita Prabhu in Santipura. The Lord stayed at Sri Advaita Prabhu's house for a few days, and knowing well that the Lord was leaving His hearth and home for good, Sri Advaita Prabhu sent His men to Navadvipa to bring mother Saci to have a last meeting with her son. His mother met Him at the house of Advaita Prabhu, and when she saw her son in sannyasa, she lamented. By way of compromise, she requested her son to make His headquarters in Puri so that she would easily be able to get information about Him. The Lord granted this last desire of His beloved mother. After this incident the Lord started for Puri, leaving all the residents of Navadvipa in an ocean of lamentation over His separation. After some time in Jagannatha Puri, Caitanya Mahaprabhu sent Nityananda Prabhu back to Bengal to get married. Bengal was known as Gauda-desa, and Lord Nityananda Prabhu overflooded this country with the transcendental loving service of the Lord.
"....If one takes shelter of Lord Sri Caitanya Mahaprabhu and Nityananda, follows Their instructions to become more tolerant than the tree and humbler than the grass, and in this way chants the holy name of the Lord, very soon he achieves the platform of transcendental loving service to the Lord, and tears appear in his eyes. There are offenses to be considered in chanting the Hare Krsna maha-mantra, but there are no such considerations in chanting the names of Gaura-Nityananda. Therefore, if one chants the Hare Krsna maha-mantra but his life is still full of sinful activities, it will be very difficult for him to achieve the platform of loving service to the Lord. But if in spite of being an offender one chants the holy names of Gaura-Nityananda, he is very quickly freed from the reactions to his offenses. Therefore, one should first approach Lord Caitanya and Nityananda, or worship Guru-Gauranga, and then come to the stage of worshiping Radha-Krsna. In our Krsna consciousness movement, our students are first advised to worship Guru-Gauranga, and then, when they are somewhat advanced, the Radha-Krsna Deity is installed, and they are engaged in the worship of the Lord."
Caitanya-caritamrta, Adi lila 8:31
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.