Dear Adwaith / Dp / Guest,
The linegae as of Gopala Sundari is not clear nor its existance in this world (i mean from which kalpam / yugam). But since many great saints Like Chidhanandar, Guhanandar, Poornanandar, Smt. Premambha (Bhanu akka - my own GURU) - all of Guhananda Mandali, have worshipped this form of Shri Vidhya Rajagopalan and attained siddhi.
People who are Sri Vaishnavas and take up Sri Vidhya upasana, (who are really a staunch Sri Vaishnava) are being initiated into this mantram. Its a boon to get the mantra which is the systhesis of Gopala n Sundari in a single form.
Also, adiyen have heard that there are many Sri Vidhya upaasakaas near the area of Kumbakonam and also are Sri Vaishnavas.
Better to take this mantra from a really competent Guru who has attained Sidhi in this form.
And as of Tnatra, please do post. Am a Kaula too. Or we could discuss the same in personal.