The teachings of Lord Krishna in Bhagavad Gita came into existence above 3650 years before now. At that time nothing like Hinduism religion existed. What existed was Sanatana Dharma (now Hinduism religion) in the erstwhile Bharatvarsha (now India).
Sanatana Dharma, a dharma existing from times eternal represents everything covered in Hinduism religion, prime reason why Hinduism is also called a way of life.
The most important aspect of Bhagavad Gita is it has universal appeal. The teachings of Bhagavad Gita are applicable to each and every single individual living on mother earth, belonging to any religion belief or faith.
Teachings of Bhagavad Gita can also be followed by anyone world over belonging to any religion, belief or faith without ever converting to Hinduism religion. One could indulge in teachings of Bhagavad Gita even while following ones religion.
The teachings of Bhagavad Gita relate to field of spirituality not religion. The spiritual journey that takes human beings inside their heart, wherein resides the spirit, our soul atman. Spirituality was all about the inner journey to reach our soul atman. Spirituality was also about demolishing our ego in totality so that one can hear dictates of Lord Krishna coming from within our heart.
Allegorically Lord Krishna is soul atman, the spirit existing in our heart. The perennial messages of Bhagavad Gita are available to mankind since birth. The moment we become egoless and can simultaneously become 100% truthful, one can distinctly hear voice of Lord Krishna from within.
The teachings of Lord Krishna in Bhagavad Gita always flowed from within our heart.