Q: Is Guru the representation of Radha Rani Jī or Balram ji?
My Reply: Those in a Nityānanda parivar will naturally feel that their gurus represent Nitai, Balarāma. Those in a Gadādhara parivar will naturally feel that their gurus represent Gadai, Rādhā. Objectively, Guru is Krishna's kṛpā-śakti. This kṛpā śakti flows through both Rādhā and Balarām, and others. The disciple will see it according to their bhāva. Neither is incorrect.
Courtesy - Vraja Kishore
The transcendental affection between Nityananda and Gadadhara can be described only by the Supreme Lord. (CB 3.7.113)
ki adbhuta pìti nityananda-gadadhare ekera apìya are sambhasa na kare
gadadhara-devera saìkcdpa ei-rupa nityananda-nindakera na dekhena mukha
nityananda-svarupere priti yara naìi dekha o na dena tare pandita-gosaìi
What wonderful affection there is between Nityananda and Gadadhara! Neither of them would ever speak to anyone who was disliked by the other.
This was the vow of Gadadhara: he would never see the face of one who offends Nityananda.
Gadadhara Pandita would avoid being seen by anyone who had no love for Nityananda Svarupa. (CB 3.7.123-125)
Text 125 indicates that Radharani would not allow anyone to enter Her camp until and unless that devotee develops love for Nitai.
gadadhara subha-drsti karena yahare se janite pare nityananda-svarupere
nityananda-svarupo yahare prita mane laoyayena gadadhara jane se-i jane
Whoever receives the merciful glance of Gadadhara can know Nityananda Svarupa.
And whoever pleases Nityananda can know Sri Gadadhara. (CB 3.7.161-162)