Is Shiva gita same as Ishvara Gita or are they different?
What is its relationship with Kurma purana?
Is Shiva gita an authoritative scripture/pramana for any shaivite sect (agamic or non-agamic sect)?
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No those texts are not the same. Shiva Gita is from the Padma Purana whereas the Ishwara Gita is found in the Kurma Purana.
Here is Shiva Gita:
The Siva-Gita is a text of Vedantic Saivism that comes to us from the latter portion (uttara kanda) of the Padma Purana. In many ways, it is a text not unlike the Bhagavad-Gita, except the focus is fully on Siva. In the Siva-Gita, the teacher is Lord Siva and the disciple is Sri Rama, and the narrator is Suta.
And, here is an abridged summary of the Ishwara Gita from the Kurma Purana.
Another link for Ishwara Gita is Kamakoti.org (Essence of Kurma Purana).