Is the Bhagavad Gita revelation? E.g. 15:15, rendered by one translator as “And I am seated in the hearts of all; from Me are memory, knowledge, as well as their loss; I am verily that which has to be known by all the Vedas; I am indeed the author of the Vedanta as well as the knower of the Vedas”.If nothing else, the authorship of Vedanta is what is revealed here. But the claim is there both to be the subject and author of revelation.
Contrary to your expectation, this question is more difficult to answer of the two. The reason is simple: the identity of the ‘I’ (in the verse) is anything but straightforward.
Does the ‘I’ refer to Krishna? On one assumption, that of the ordinary English language, the answer appears to be ‘yes’. In that case, the verse is false on all counts: Krishna was born after the transmission of the Veda’s began (so he cannot be the author), and he cannot be ’the knower’ (with an emphasis on ’the’). After being dead for ages, he is no way responsible for either memory, knowledge or their loss.
If we say that the ‘I’ does not refer to Krishna, the empirical person, we need to say what else does it refer to. In English language, the ‘I’ is an indexical: its reference varies according to the place, time and person. Even if it refers to something called the ‘Krishnahood’ (the essence of Krishna), such an essence helps us identify the unique person that Krishna is unerringly. (We know who we are talking about, when I say ‘If Krishna were to be born in Belgium today, he would be a Belgian citizen’. These kinds of sentences are understandable if and only if the reference of Krishna is unique and does not vary.)
In other words, the ‘I’ in English is a guarantee for the uniqueness of the person uttering the indexical. In the Indian traditions (especially in the adhyatmic contexts), that is not the case. The ‘I’ does not make us into individual persons but picks something out that is present in each entity capable of self-reference. It is only thus that we understand the beginning of the sentence in the translation: “I am seated in the hearts of all”. Krishna can be this ‘I’ only in the sense that each organism has an ‘I’. Indians do not ascribe the status of an indexical to this ‘I’, even if it is guaranteed an invariant reference.
However, the previous point can be elaborated in any number of different ways: from the Jain ‘I’ to the Advaita ‘I’; from the Dvaita ‘I’ to the Buddhist ’non-I’. These multiplicities are also partially dependent on the nature of experiencing the ‘I’. The problem in that context is also one of elaborating the identity of the experiencer: who does the experiencing?
In short, picking up a verse from the Gita or any other verse from other texts creates enormous interpretative complications. Such a task is enormously complex compared to answering the question, ‘What does apaurusheya mean?’