I don't know which version or edition of Padma Purana has this composition of Shiva Gita. It also looks like none of the acharyas before 15th century have written any commentary on this text. 

The only reference to any kind of a timeline I have seen is only [here][1]. Sri Abhinava Nrisimha Bharati (1600 – 1623) of Sringeri Sharada Peetham, seems to have written a commentary on Shiva Gita.

> An expert in mantrasastra, Sri Abhinava Nrisimha Bharati was an adept
> of a high order. A commentary on the Shiva Gita, that he wrote, is an
> outstanding work.

The only available *translation* of this text I have seen online is by a Santosh Kumar Ayalasomayajulu, found [here][2]. But even in that text, there is no actual reference to the parent text it was derived from (apart from simply saying uttara khanda of padma purana). And it also doesn't mention which acharya's commentary his translation is based on. So, I am not inclined to take it as an authoritative version. Curiously, the same translated text is also found [here][3]. 

You may also look at this question on [this][4] very site. It looks like not much material is available on or about that text.

  [1]: http://www.sringeri.net/jagadgurus/the-later-acharyas-2
  [2]: https://archive.org/details/SrimadShivaGita
  [3]: http://www.vrindavana.net/academy/wp-content/uploads/2016/01/SivaGita-I.pdf
  [4]: https://hinduism.stackexchange.com/questions/11317/shiva-gita-in-english-with-commentaries-from-eminent-philosophers