Swami Vivekananda makes a remark in one of his lectures that many Vaishnava theories have come from Gopala Tapini Upanishad.
Complete Works / Volume 3 / Lectures from Colombo to Almora / The Religion we are born in:
If any sect in India wants to have its ideas established with a firm hold on the people it must base them on the authority of the Vedanta. They all have to do it, whether they are Dvaitists or Advaitists. Even the Vaishnavas have to go to Gopâlatâpini Upanishad to prove the truth of their own theories.
There are a lot of Vaishnava scriptures which Vaishnavas refer frequently so which theories are unique to Gopalatapini Upanishad which are not found in other Vaishnava scriptures?