First let me point out that to me only the original sanskrit slokas are 'authentic' Gita.
Now I come to the 'Authentic' commentaries.So far as I know, the first authentic commentary was written by Sri Adi Sankaracharya, which is the bhashya that establishes the Advaita philosophy.He made commentary of the 700 slokas of Gita out of 745 slokas. All the later Acharyas like Sri Ramanuja, Sri Madhva etc wrote commentaries only of these very 700 slokas chosen by Sri Adi Sankara. The purpose of each was to establish one's own philosophy by explaining Gita.They wrote the bhashyas in sanskrit, analysyiing the slokas word by word and going to the very depth of each word. Sanskrit is a very very rich language and each word can be interpreted in so many different ways.But for all the Acharyas, the target readers were sanskrit scholars of their own and the other sects, and so besides being unthinkably knowledgeable in sanskrit, they were very keen on maintaining the standard of their commentaries. Each Acharya after Sankaracharya had to analyse in detail how each sloka supports the very philosophy preached by him in essence.
ISKCON's Gita on the other hand is written in English. Its objective is to preach Sri Krishna as the supreme God primarily to the residents of the foreign countries. So it is quite obvious that the depth and beauty that was there in the bhashyas of the Acharyas written to influence the scholars is not expected from this version. Here, the attempt is to establish a monotheistic type of religion that would appeal the followers of the other religions. So Sri Krishna has been projected as 'The Supreme Personality' of Godhead and other gods branded as demi-gods. 'god' versus 'Godhead' was not found enough to express the insignificance of other deities as compared to Sri Krishna,and so He was made 'The Supreme Godhead' (Parama- Ishwara ?) while other deities 'Demi-god' (Upa-Devata/Apa-Devata). No bhashyakara Acharya had the need to use such words and this is one big difference.
Another difference with the Udbodhana version of Gita providing relevant parts from the commentaries of Sri Adi Sankara, Sri Sridhara Swami and Sri Madhusudan Sraswati(this 'Udbodhan'-Gita is read widely in Bengal and is personally very dear to me) are also worthy of mention. One beauty of the Gita slokas is its synthesis of the four yogas (Karmayoga, Raja-yoga, Jnana-Yoga and Bhakti-yoga) as mentioned by Swami Vivekananda. The names of the different chapters also make it very clear. So, Sri Krishna admits that the Supreme Goal can be reached by all these different paths. Yes, lastly He asks Arjuna to surrender to Him, which can be termed the 'Saranagati-yoga', but He keeps other paths reaching Him fully open. The ISKCON-Gita on the other hand converts all other yogas to bhakti-yoga--- because this version is written to preach 'Krishna-consciousness'.
The above had led to many startling interpretations, I am mentioning just one of them here, which becomes clear from a conversation between Bhaktivedanta Swami (here referred to as 'Prabhupada') and some Asok Chugani (Reference :https://vedabase.io/en/library/transcripts/770108r2bom/?query=Bhagavad+Gita+14.27#bb424147) :
Ashok Chugani: Swāmījī, may I ask one question which I had much discussion with your śiṣya here. Brahman, the sort of, what we in Hindu philosophy consider as the ultimate Godhead...
Ashok Chugani: Brahman you consider Kṛṣṇa. But He is, of course... He was incarnate. He was one of the avatāras.
Prabhupāda: That is your opinion. [laughter] That is not...
Ashok Chugani: That is our point which I want to clarify.
Prabhupāda: You have to take the śāstra, authority.
Ashok Chugani: Does not Gītā say...
Prabhupāda: Brahmaṇo aham pratiṣṭhā. Read Bhagavad-gītā. You have read Bhagavad-gītā? .. Brahmaṇo 'haṁ pratiṣṭhā (14/27) Read. Just here. So He is the pratiṣṭhā. Just like in this room there is illumination. The illumination is spread all over the room...So the illumination is important or the light is important? What is important?The impersonal Brahman illumination, or effulgence, that is the bodily rays of Kṛṣṇa.
The Udbodhan-Gita obviously does not explain the sloka in this way. It offers two alternative explanations --both from the commentary of Sri Adi Sankaracharya ---of the sloka (Gita, 14/27):
Brahmano hi pratisthaa aham amritasya avyayasya cah/shasvatasya cha dharmasya sukhasya aikantikasya cha//
I am quoting both below (translated from Bengali. Anyone is free to demand a photo-copy of the original pages).
(a) I (the Pratyagaatma) am changeless (Avyaya), Deathless (Amrita),Eternal (Sanatana), Reachable by religious path of the Jyana-yoga, and am the source of Bliss of the Paramatman.[ by samyak jnana, the pratyagaatma becomes fully convinced that It is one with the Paramatman.]
(b) I (the Formless Brahman) am the abode(Pratistha) of the Deathless, Changeless, Saguna Brahma and so I am the essence of the Sanatana Dharma in form of the undisturbed dedication to Jnana-marga, am the abode of the Supreme Bliss also. (page 321-22).
I am attaching the explanation of the sloka by Krishnaprem, the siddha Gaudiya Vaishnava from His 'The Yoga of the Bhagavad Gita(Penguin, 1937) in image form.
The rays are here coming to me from the depth and the beauty of explanations.