Gaudiya literature such as the Śrī Caitanya-caritāmṛta sheds light on Guru Tattva, the ontological position of the spiritual master.
In CC 1.1.44/45 we find the following:
yadyapi āmāra guru — caitanyera dāsa
tathāpi jāniye āmi tāṅhāra prakāśa
guru kṛṣṇa-rūpa hana śāstrera pramāṇe
guru-rūpe kṛṣṇa kṛpā karena bhakta-gaṇe
Although I know that my spiritual master is a servitor of Śrī Caitanya, I know Him also as a plenary manifestation of the Lord. According to the deliberate opinion of all revealed scriptures, the spiritual master is nondifferent from Kṛṣṇa. Lord Kṛṣṇa in the form of the spiritual master delivers His devotees.
Furthermore, Śri Hari-bhakti-vilasa 4.346-365 quotes a variety of scriptures where the Guru is equated with the Lord Himself. Examples are "know the acarya as Myself" (SB 11.17.27 ācāryaṁ māṁ vijānīyān, quoted in 4.437), "the Guru is Viṣṇu" (4.352) and "He who is known as the Guru is known as Hari" (yo gurūḥ sa hariḥ smṛtaḥ, quoted in 4.353). In Bhakti Sandarbha (202, 211, 213), Jīva Gosvāmī also raises the topic of the nature of the Guru in relation to God.
All works mentioned above are native to the Gaudiya tradition and I was hard-pressed to find any writings of members of other Vaiṣnava traditions which would address the issue at play. Are there any works of non-Gaudiya Vaiṣnavas who elaborate on Guru Tattva (whether the Guru is as good as God or God Himself)?