Ishvara Gita of the Kurma Purana, dedicated to Lord Shiva, is very similar to the Bhagavad Gita. However, it is not very popular as the Bhagavad Gita. Now my question is that did Adi Shankara ever write anything about the Ishvara Gita in any of His works?
No, Adi Shankaracharya hasn't written anything on Ishvara Gita.
There are the list of his works including his commentaries, stotras in Kanchi Kamakoti peetham official website. We won't find anything about Ishvara Gita in the Kurma Purana.
There is another website named advaita-vedanta.org.
Even in this website, there is no mention of it.
In his commentary to the Brahma sutras 2.3.45, he has mentioned the word Ishvara Gita.
ईश्वर गीतास्वापि च इष्वरांशत्वं जीवस्य स्मर्यते
After saying this, he quoted the Bhagavad Gita verse 15.07.
ममैवांशो जीवलोके जीवभूतः सनातनः
So, it is clear that Adi Shankaracharya meant Bhagavad Gita by Ishvara Gita and not the gita from the Kurma Purana.
So, it is safe to conclude that he has not written anything about Ishvara Gita also he meant Bhagavad Gita by writing Ishvara Gita. The only Gita he has commented and quoted is Bhagavad Gita.