CC Antya 2.31
aviśvāsa chāḍa, yei kariyācha antara”
“You are chanting the Gaura-gopāla mantra, composed of four syllables. Now please give up the doubts that have resided within you.”
Śrīla Bhaktivinoda Ṭhākura explains the Gaura-gopāla mantra in his Amṛta-pravāha-bhāṣya. Worshipers of Śrī Gaurasundara accept the four syllables gau-ra-aṅ-ga as the Gaura mantra, but pure worshipers of Rādhā and Kṛṣṇa accept the four syllables rā-dhā kṛṣ-ṇa as the Gaura-gopāla mantra. However, Vaiṣṇavas consider Śrī Caitanya Mahāprabhu nondifferent from Rādhā-Kṛṣṇa (śrī-kṛṣṇa-caitanya rādhā-kṛṣṇa nahe anya). Therefore one who chants the mantra “gaurāṅga” and one who chants the names of Rādhā and Kṛṣṇa are on the same level.