Sri Caitanya Mahaprabhu took initiation from Sri Isvara Puripada, who first gave Him harinama, and to purify that chanting of nama, he gave Him gopal-mantra. Gopal-mantra is transcendental and ever-existing, and it gives a relationship with Krsna as Gopijana-vallabha.
Sri Caitanya Mahaprabhu took initiation even though he is the Lord of lords – Radha and Krsna combined – and yet He went to Gaya and took initiation. He vividly explained this fact in a gathering of sannyasis in Kasi when those sannyasis asked Him, “O, You are a very young and qualified sannyasi. Why do you not hear Vedanta? Why do you engage in singing and dancing and rolling on the ground? A sannyasi should not dance and do all these things.” Sri Caitanya Mahaprabhu replied by telling them of the order of His guru, Sri Isvara Puripada:
Mahaprabhu said, “Guru mora murkha dekhi – My Guru Maharaja told me that because I am foolish and unintelligent, I am not qualified to study Vedanta. He said that instead I should always chant Hare Krsna and I should remember this verse:
kalau nasty eva nasty eva nasty eva gatir anyatha
(Caitanya Caritamrta Adi-lila 17.21)
[“In this Age of Kali there is no other means, no other means, no other means for self-realization than chanting the holy name, chanting the holy name, chanting the holy name of Lord Hari.”]
But Sri Caitanya Mahaprabhu not only took harinama from His gurudeva; He also took second initiation through the gopal-mantra. In our parampara, in order to purify one’s chanting and to and give the chanter a relationship with Krsna who has kindly taken the form of His holy name, the gopal-mantra is given along with harinama.