GeetA is considered to be the "moksha shAstra" mainly by the vedantins and they have also conferred it the title or Gitopanishad.
It also forms the core of Vedanta as it is the part of the prasthana trayi of the Vedanta. Hence gItA is believed to be the greatest of moksha shastra as it focuses on mukti/liberation.
As per Geeta none before Arjuna have ever seen the divine Vishvaroopam of Lord Krishna. So Arjuna was the lucky one who witnessed this for the first time ever.
However, as per the mahAbhArata, the pAndavas including Arjuna died in the himAlayas and ascended to hell and heaven respectively.
This is mentioned in swargArohana parva of MahAbharata,
"Yudhishthira, however, was exceedingly disconsolate and stupefied by the foul odour. Resolved to return, O Bharata, he retraced his steps. Afflicted by sorrow and grief, the righteous-souled monarch turned back. Just at that moment he heard piteous lamentations all around, ‘O son of Dharma, O royal sage, O thou of sacred origin, O son of Pandu, do thou stay a moment for favouring us. At thy approach, O invincible one, a delightful breeze hath begun to blow, bearing the sweet scent of thy person. Great hath been our relief at this. O foremost of kings, beholding thee, O first of men, great hath been our happiness. O son of Pritha, let that happiness last longer through thy stay here, for a few moments more. Do thou remain here, O Bharata, for even a short while. As long as thou art here, O thou of Kuru’s race, torments cease to afflict us.’ These and many similar words, uttered in piteous voices by persons in pain, the king heard in that region, wafted to his ears from every side.
"Hearing those words of beings in woe, Yudhishthira of compassionate heart exclaimed aloud, ‘Alas, how painful!’ And the king stood still. The speeches of those woe-begone and afflicted persons seemed to the son of Pandu to be uttered in voices that he had heard before although he could not recognise them on that occasion.
"Unable to recognise voices, Dharma’s son, Yudhishthira, enquired, saying, ‘Who are you? Why also do you stay here?’
"Thus addressed, they answered him from all sides, saying, ‘I am Karna!’ ‘I am Bhimasena!’ ‘I am Arjuna!’ ‘I am Nakula!’ ‘I am Sahadeva!’ ‘I am Dhrishtadyumna!’ ‘I am Draupadi!’ ‘We are the sons of Draupadi!’ Even thus, O king, did those voices speak.
"Hearing those exclamations, O king, uttered in voices of pain suitable to that place, the royal Yudhishthira asked himself ‘What perverse destiny is this? What are those sinful acts which were committed by those high-souled beings, Karna and the sons of Draupadi, and the slender-waisted princess of Pancala, so that their residence has been assigned in this region of foetid smell and great woe? I am not aware of any transgression that can be attributed to these persons of righteous deeds. What is that act by doing which Dhritarashtra’s son, king Suyodhana, with all his sinful followers, has become invested with such prosperity? Endued with prosperity like that of the great Indra himself, he is highly adored. What is that act through the consequence of which these (high-souled ones) have fallen into Hell? All of them were conversant with every duty, were heroes, were devoted to truth and the Vedas; were observant of Kshatriya practices; were righteous in their acts; were performers of sacrifices; and givers of large presents unto brahmanas. Am I asleep or awake? Am I conscious or unconscious? Or, is all this a mental delusion due to disorders of the brain?’
Now this raises a big question,
Why did not arjuna get moksha despite the great moksha upadesha from the lord himself?
His death and gati after death was pretty much like other mortals.
From the above mentioned scenario, there can be two scenarios,
1. Arjuna did not practice the given upadesham
2. The upadesham was not competent enough (which is unlikely as it was from Lord Krishna himself!).
But as per scenario 1. if arjuna did not intend to practice the spiritual discipline then why was he given upadesham at all? if he was not the sadpatra for atma vidya?
If he had practiced the upadesham in the precise manner then how come he not attain moksha?
Or was some chapters of gita were interpolated as asked in this question?