Pandava Gita is a stotra or a hymn dedicated to Lord Krishna or Lord Vishnu. There are 76 verses sung by many including Lord Brahma, sages, Indra, Vaishampayana, Pandavas, Draupadi and also Duryodhana, Karna, Gandhari etc., praising or worshipping Lord Krishna.
All these verses are taken from different situations sung by different people, rishis, Devatas.
On Hindupedia.com, there is a note added by translator P.R Ramchander while translating this stotra.
Here is an anthology of mellifluous quotations drenched in devotion, in which many characters of the epic Mahabharatha figure as authors. Most of these prayers are not there in the Mahabharatha epic. The existence of such an anthology is mentioned in the Bruhath Stotra Sagara. . Two versions of the Pandava Gita were available to me. One of them is from Shri Chitrapur Math, Shirali (Karnataka ) (http://mahabharata-resources.org/) and the other is a publication by Swarnapuri publication from Tamil Nadu, in which it is told that the text has been collected from Andhra Pradesh. While the Chithrapur collection has 82 slokas including Phala Sruthi, the swarnapuri collection has 91 slokas. While Six slokas of Chithrapur collection does not find their place in the Swarnapuri collection, 15 slokas of the Swarnapuri collection do not find their place in the Karnataka collection. The translations and transliterations in simple roman script are mine own and I acknowledge my gratefulness to the Chithrapur collection and the Swarnapuri collection.)
This English translation of Pandava Gita is the version of Chitrapuri math the translator is talking about in the note.
As said by him in the note, many shlokas are not found in the Mahabharata. They could be in other Puranas too. This suggests that it is a collection of verses and compiled later by some person. As there are mentions of Pandavas and Kauravas in it, it is a post-Mahabharata text.
My question is who composed Pandava Gita into current form? What is the earliest reference to it by anyone?