Bhagavad Gita, a purely spiritual treatise does not deal with topic of success. Bhagavad Gita relates to inner journey of the spirit, consciousness existing in our heart as soul atman.
Human beings who relate Bhagavad Gita to success were followers of religion (rituals), something that has nothing to do with spirituality.
What kind of success can Bhagavad Gita indulge in? It is following precepts of spirituality contained in Bhagavad Gita Mahavira, Gautama Buddha even Jesus Christ and Prophet Mohammed finally succeeded in reaching state of enlightenment. Sri Ramakrishna Paramhansa and Maharshi Ramana also succeeded in reaching God following precepts of spirituality contained in Bhagavad Gita.
As per Bhagavad Gita real success in life is achieved when we indulged in spirituality not religion. Reason being we are primarily a spiritual traveler on its cosmic journey that manifest human form to work out its karma, remove dross impurities within. The ultimate goal of life as a human being and also as a soul atman is emancipating self from cycle of birth and death forever.
Becoming a pure soul atman is the cosmic goal of life for every human being on mother earth.
We should never try to mix spirituality with day-to-day routines of life, the material world. Both have different goals of life. One indulging in religion (rituals) can have any goal in life related to material world but goal of a spiritual traveler always is reaching 8.4 millionth manifestation. Reaching God, state of self realization, god realization is our ultimate destination.
What use of mortal success when after death of body, one keeps rotating in endless cycles of birth and death again and again. If we desire ending this, our indulgence in spirituality detailed in Bhagavad Gita is an absolute must. Ultimate success is completing our cosmic journey and reaching abode of God (kingdom of God) called Vaikuntha in Hinduism.