Can anyone tell me why do Gaudiya Vaishnavas believe in Brahma Samhita?
Anyway, if I may, I would like to explain our reasoning behind why we accept unorthodox scriptures like the Brahma Samhita. There is unfortunately a lot of misconceptions about Gaudiya Vaishnavism and this is soiling the view the Hindu community has of it.
First of all, in Hinduism there are a lot of sects and sampradayas and they all have, at times, quite differing beliefs. This is because the acharyas of those sects use different shastras in order to support their siddhanta. The categorization of the Puranas, for example, as sattva,raja and tamo and the verses that contribute to this are accepted by the Vaishnav schools, but rejected by the others. Some Hindu Sampradayas accept only the Karma Khanda portions of the Vedas and reject the Upanishads. Even in the study of Vedanta, there is a lot of difference. The traditional 3 schools of Vedanta (Sankara, Ramanuja and Madhavacharya) focus on the prasthana-trayi, while we Gaudiyas for example, focus on Srimad Bhagavatam (as we interpret that to be the essence of Vedanta).
What, in essence, I am saying is that even Vedic authority derives itself from the Guru Parampara. It is the foundation of all knowledge, because the Guru (and acharyas) tell us which scripture is authentic and which are interpolated. That is why, we Gaudiyas accept Brahma Samhita, because it was a shastra which was believed by our acharyas (like Jiva Goswami and Bhaktivinoda Thakur) to be authentic. In Chaitanya Charitamrta it is told that Lord Chaitanya traveled to South India (Tamil Nadu) and found a copy of one chapter of the Samhita and copied it by hand. So Brahma Samhita is authentic to us, and the evidence is because Lord Chaitanya and our Acharyas accept it. In-fact this is the case for many if not all Sampradayas. For example one may raise the question; "Why should we accept the Prasthana Trayi? Such a concept is in-fact not found anywhere in Sruti or Smrti" The answer to that of-course is we accept Prathana Trayi because we have faith in the words of Acharyas like Sankaracharya and Ramanunajacharya who accepted them also. The source of all knowledge is in actuality faith in the Acharyas. Trying to approach the scriptures in any other way, will often lead a student to great confusion. That is why Sruti declares:
yasya deve parā bhaktir yathā deve tathā gurau
tasyaite kathitā hy arthāḥ prakāśante mahātmanaḥ
Only unto those great souls who have implicit faith in both the Lord and the spiritual master are all the imports of Vedic knowledge automatically revealed. (Śvetāśvatara Upaniṣad 6.23)
Gaudiya Vaishnavas believe Lord Krishna is the Supreme Brahman and Brahma Samhita also describes the same.
īśvaraḥ paramaḥ kṛṣṇaḥ sac-cid-ānanda-vigrahaḥ
anādir ādir govindaḥ sarva-kāraṇa-kāraṇam
Translation: Kṛṣṇa who is known as Govinda is the Supreme Godhead. He has an eternal blissful spiritual body. He is the origin of all. He has no other origin and He is the prime cause of all causes.
The above verse describes Lord Krishna is the origin of everything and all the other verses glorifies Him.
Why do they Gaudiya Vaishnavas believe in Brahma Samhita?
Because Brahma Samhita is one of the Pancharatra texts which are most popular among Vaishnavas.
Here is the list of Pancharatra agamas from wiki:
Agastya-Samhita, Aniruddha-Samhita, Ahirbudhnya Samhita, Brahma Samhita, Brihat-Brahma-Samhita, Isvara-Samhita, Kapinjala-Samhita, Garga Samhita, Gautama-Samhita, Citrasikhandi-Samhita, Jayakhya-Samhita.... (mentioned few from list)
It is also believed that there were 100 chapters in the Brahma Samhita. This 5th chapter is the only chapter available now and was discovered by Chaitanya.