The Shiva Gita (which was part of the Padma Purana, as per a few sources) has a lengthy discourse, between Shiva and Rama on moksha shaastra, like the Bhagavadgita.
The Shiva gita, a discourse on Vedanta between Shiva and Sri Rama, said to be in the Padmapurana but not available there in the present times, is cited, naming it, by Sripati Panditacharya of the 11/12th Century CE in his Brahma sutra Srikara Bhashya. Whatever be the date of Srikara Pandita, that he has cited the verses by naming the source is an important evidence.
The Narada Purana and another purana do say that the ShivaGita is part of the Padma purana.
There are several commentaries to the ShivaGita.
The verses cited in the Srikara Bhashyam are indeed found in the present day ShivaGita:
In this chapter, it says that Moksha, liberation, is not travelling to any destination but the state of being free from ignorance which had caused bondage:
शिवगीता/त्रयोदशोऽध्यायः
https://sa.wikisource.org/s/1j9z
यदा सर्वे प्रमुच्यन्ते कामा येऽस्य वशं गताः ।
अथ मर्त्योऽमृतो भवत्येतवदनुशासनम् ॥ ३१ ॥
मोक्षस्य नहि वासोऽस्ति न ग्रामान्तरमेव वा ।
अज्ञानहृदयग्रन्थिनाशो मोक्ष इति स्मृतः ॥ ३२ ॥
वृक्षाग्रच्युतपादो यः स तदैव पतत्यधः ॥
तद्धज्ज्ञानवतो मुक्तिर्जायते निश्चितापि तु ॥ ३३ ॥
The Balanandini commentary says: वैराग्यपूर्वकं ज्ञानोत्पत्तौ सत्यां न मोक्षे विलम्ब इत्याह – मोक्षस्येति । मोक्षस्य वासोऽवस्थानं नास्ति कैलासवैकुण्ठादाविति शेषः । ग्रामाद्ग्रामान्तरं प्रति गमनं यथा तथा मोक्षस्य गमनं नास्ति । तहिं को नाम मोक्षस्तत्राह । अज्ञानं तत्कार्य- हृदयग्रन्थिरहमिति तादात्म्याध्यासरूपश्च, तयोर्नाश एव मोक्ष इत्यर्थः ॥ ३२ । तत्र दृष्टान्तमाह – वृक्षाग्रेति । यथा वृक्षाग्रात् च्युतपादस्य भूमिप्राप्तौ न विलम्बस्तथा ज्ञानिनो मुक्तावपि न विलम्ब इति भावः ॥ ३३ ॥
An analogy is given: Just as a person on a tree slips and in no time lands on the ground, so too the direct realization of the self marks the state of liberation.
Yet, it is not a state either, for Shankaracharya has explained that it is the realization that one was never ever bound.
The Bhagavadgita puts it as:
नान्यं गुणेभ्यः कर्तारं यदा द्रष्टानुपश्यति ।
गुणेभ्यश्च परं वेत्ति मद्भावं सोऽधिगच्छति ॥ १९ ॥ 14.19
One realizes the state of Brahman
मां च योऽव्यभिचारेण भक्तियोगेन सेवते ।
स गुणान्समतीत्यैतान्ब्रह्मभूयाय कल्पते ॥ २६ ॥ 14.26
One becomes fit to ‘become’ Brahman.
इति क्षेत्रं तथा ज्ञानं ज्ञेयं चोक्तं समासतः ।
मद्भक्त एतद्विज्ञाय मद्भावायोपपद्यते ॥ १८ ॥
The devotee of the Lord becomes fit to become Brahman.
The Bhagavatam 2.10.6 too says:
मुक्तिर्हित्वान्यथारूपं स्वरूपेण व्यवस्थिति: ॥ ६ ॥
The giving up of the notion that one is a jiva, and remaining over as one’s true nature, is what is called mukti, liberation.
In the Mundakopanishad 3.2.6 bhashya Shankara says:
परामृताः परम् अमृतम् अमरणधर्मकं ब्रह्म आत्मभूतं येषां ते परामृता जीवन्त एव ब्रह्मभूताः, परामृताः सन्तः परिमुच्यन्ति परि समन्तात्प्रदीपनिर्वाणवद्भिन्नघटाकाशवच्च निवृत्तिमुपयान्ति परिमुच्यन्ति परि समन्तान्मुच्यन्ते सर्वे, न देशान्तरं गन्तव्यमपेक्षन्ते । ‘शकुनीनामिवाकाशे जले वारिचरस्य वा । पदं यथा न दृश्येत तथा ज्ञानवतां गतिः’ (मो. ध. १८१-९) ‘अनध्वगा अध्वसु पारयिष्णवः’ (?) इति श्रुतिस्मृतिभ्याम् ; देशपरिच्छिन्ना हि गतिः संसारविषयैव, परिच्छिन्नसाधनसाध्यत्वात् । ब्रह्म तु समस्तत्वान्न देशपरिच्छेदेन गन्तव्यम् । यदि हि देशपरिच्छिन्नं ब्रह्म स्यात्, मूर्तद्रव्यवदाद्यन्तवदन्याश्रितं सावयवमनित्यं कृतकं च स्यात् । न त्वेवंविधं ब्रह्म भवितुमर्हति । अतस्तत्प्राप्तिश्च नैव देशपरिच्छिन्ना भवितुं युक्ता ॥
The liberated do not travel to any other place/loka, for any such travel will imply that the jiva is still in samsara. Since Brahman is infinite, the jnani who has realized his identity with Brahman, also being the Infinite Brahman alone, does not go anywhere upon death. For, Brahman is not a finite place to be reached/attained. If Brahman were located in a place then Brahman, being no different from any formed object, will have to have a beginning and end, and be dependent on something else, be made of parts, and ephemeral, and a produced one. Brahman can never be of this nature. Thus, the ‘attainment’ of Brahman cannot be involving any locating in some other place.
Om Tat Sat