It is a perfect saying that lessons of Bhagavad Gita are immortal, for it is following the teachings contained in Bhagavad Gita human beings finally reached state of immortality forever.
Immortality cannot be gained in absence of teachings contained in Bhagavad Gita. In present Kali Yuga the only recourse to understanding spirituality that led mankind towards state of enlightenment laid in teachings of Bhagavad Gita.
Gaining immortality, becoming a pure soul atman is the ultimate goal of human life on mother earth. Every single human being on mother earth one way or other shall indulge in spirituality to reach cosmic end of life, 8.4 millionth manifestation.
If we are wise we shall attempt it in present life time. Otherwise we shall keep rotating in endless cycles of birth and death. We may be as famous as Bill Gates or Warren Buffett but to what avail, both shall keep rotating in endless cycles of birth and death after death of present body.
To gain emancipation from cycle of birth and death, indulgence in spirituality contained in Bhagavad Gita is the only solution and to follow contents of Bhagavad Gita our mastering art of contemplation (chintan) is an absolute must. As nothing in Bhagavad Gita is understood literally, we had to indulge in contemplation and had no other alterative.
Furthermore it is Bhagavad Gita that tells us about our cosmic journey of life. As per Bhagavad Gita we are primarily a spirit, a cosmic traveler manifesting human form. Preloaded with an ego since birth, we failed to realize our true identity. It is only by indulging in spirituality contained in Bhagavad Gita we finally realized our true inner self.
Spirituality contained in Bhagavad Gita is all about undertaking inner journey towards the spirit existing in our heart. Indulgence in pranayama, self enquiry and also Neti (not this, not this) is also for this purpose. The prime purpose of breathing exercise is reaching the spirit existing in heart.
All forms of yoga meditation had only one purpose in life, reach state of absolute purity as a soul atman. Indulging in 700 shlokas verses contained in Bhagavad Gita we can easily achieve our goal of life, become a pure soul atman forever.
Immortality is never gained in physical form. Immortality always belonged to soul atman within. Indulging in spirituality detailed in Bhagavad Gita when human beings finally reached state of enlightenment, the soul atman within gained absolute purity. This liberation from cycle of birth and death by a soul atman is termed state of immortality.
A soul atman becomes immortal moment it regains its lost original pure prime pristine primordial form.