The Gita is a poem, in fact, an excerpt from a poem. It is not sruti.
The Gita does sum up very succinctly many of the general tenets of Sanatana Dharma and, thus, has become accepted into the body of smriti.
Hinduism is a chaotic crazy-quilt of tradition and philosophy. It has no formal canon or catechism. Perhaps the sole, essential feature distinguishing it from western religions is the concept of Brahman -- an undifferentiated, featureless Universal Consciousness/Reality underlying all of our subjective perceptions.
Whilst a Jnani intellectual may be able to relate to The Universe as mathematical equation; as Superstring and Relativity theory (Advaita Vedanta), most people's spiritual progress is best served by concentrating on an Aspect of the Godhead.
Sri Krsna is a perfectly serviceable Aspect upon which to concentrate one's devotion, if a bhakti, but Lord Siva, Ganesa or Kali are equally conducive to spiritual progression if one can more comfortably relate to their qualities.
We must remember that the worship of Personalities and Avataras is a crutch, a therapeutic modality useful in our journey toward Unity with Brahman.
Insistence on one Aspect as objectively real to the point of contention with dharmis of other traditions is counter to the essence of Sanatana Dharma.
