Gītā as smrti-granthah is a composition which comprises of remembrance or recollection of the teaching of śruti / Upanishad, unfolds the truth of our true nature by pointing out our problems / life's dilemma through Arjuna's experience. This is presented in the first chapter titled Arjuna-vishāda (arjuna's sorrow).
Lord Krsna imparted the teaching of the true nature of the self to Arjuna as the solution not just for Arjuna, but mainly for entire humanity. Even though each of us is having different problem, but the cause is the same, which is not knowing our limitless nature. The knowledge of myself being Sat (existence) which never ceases to be, is to dismiss our fears towards death either of our own body or of others. I am Cit (consciousness / knowledge) as the basis of the entire world, making me as self-evident self and making the world becomes evident to me. Just like my own dream is created by my own knowledge and being acknowledged by me. If I am ignorance of dream being dream which is my own projection alone, then all short of situation in the dream will create emotional torment for me. Only by waking up from this ignorance that I am alone is the basis for this entire world, then none of the situation will affect me, just like dream will not affect the waking person. Therefore what is me is only Ānanda, unsurpassed happiness / fullness, because I am not limited by place, time and other entities. I am the limitless-self, nothing in the world would limit me. This truth of me is presented in second chapter titled sānkhya-yoga, means ātma-jñāna - self knowledge.
This knowledge can be known only through the path of jñāna-yoga, where two lifestyles can be adopted for this pursue. One lifestyle is through the life of sannyāsa (monkshood) and another is karma-yoga (active life in society). We are advised to take up the path of karma-yoga to qualify ourselves for this knowledge. There were a lot of qualification talked about for sannyāsī (monk) and grhastha (house-holder) through out chapter two and three (with the main theme on karma-yoga). But most important is the spirit of karma-yoga which is doing action as offering to the Lord by following the law of karma including doing one's own duty, and receiving the result of action with gratitude. This karma-yoga spirit culminates to akārta (non-doer) and abhokta (non-experiencer) with the support of ātma-jñāna. Only by knowing that I am sat-cit-ānanda ātma is akārta (non-doer) / abhokta (non-experiencer), then one can be free from actions and their results. Just like in chapter 3 verse 5 said that physical non-action is impossible because in this transactional world, all are made to act by guna (qualities) on the manifestation. Manifestation itself is a constant change, and for changes to take place, action ought to be there. Therefore one can be totally free from the realm of samsāra (life of becoming), only by knowing (cognitively) my true nature itself never bound / tainted by any action, because action is only interaction between body-mind-complex with their respective objects (Gītā 3-28). I the saksi (witness) is ever free from action. This is possible only by knowledge of the self. This is the central theme of fourth chapter jñāna-karma-sannyāsa-yoga (renunciation of action through knowledge).