What is the central theme of the Bhagavad Gītā ? There are several ways to approach this question. We present three perspectives as follows:
Conjoining the last word of the last verse of the Gītā (18.78) मम (mama) with the first word of the first verse of the Gītā (1.1), धर्म (dharma), we get ‘मम धर्म,’ denoting ‘my duty.’ And if we rearrange the first two words of the Gītā dharmakṣetre kurukṣetre ‘धर्मक्षेत्रे कुरुक्षेत्रे’ as ‘क्षेत्रे क्षेत्रे धर्मम् कुरुः’ (kṣetre kṣetre dharmam kuruḥ), we get the following meaning: In every sphere of life, do the right thing—in the full wakefulness of Self-knowledge.
Another perspective in approaching the timeless message of the Gītā is to look at first and last words of Śrī Kṛṣnạ’s teachings in the Gītā (2.11 and 18.66). The very first word of the teaching is aśocyāna (अशोच्यान 2.11)—not worthy to be grieved for; and the last word is mā śucaḥ (मा शुचः 18.66)—do not grieve. The first śloka (2.11) further declares that the wise do not grieve (नानुशोचन्ति पण्डिताः). The Gītā expounds the nature of the essential nature of the Self as immutable, eternal, and imperishable. The idea is that the wise, knowing the Self as such, have no reason to grieve (2.25-27, 29; 18.54). Thus, the Gītā offers the highest security that there is in the form of Self-knowledge.
Śrī Rāmakrishna, the great seer-saint of modern India, used to say that one could understand the essential meaning of Gītā by repeating the word ‘Gītā, Gītā Gītā Gītā.’ If the word Gītā is repeated several times, one finds oneself uttering ‘ta-Gi ta-Gi ta-Gi…,’ and it comes to sound like tāgi [tyāgi–one who renounces].
This is the teaching of the Gītā– ‘Realize the Real by giving up the unreal.’
In the Gītā’s view, the performance of actions selflessly as a service to the Supreme purifies the mind and makes it a fit vessel for the reception of Self-knowledge which alone is the means to spiritual freedom. Śrī Kṛṣṇa, the teacher Par Excellence in the Gītā, does not merely want to make us philosophically learned but to help us realize the Truth experientially—not to merely instruct but to make us truly wise and free.
The wisdom of the Gītā is available to everyone who is interested in it. It transcends all distinctions and boundaries. It is humbly offered in the spirit of ‘to whom it may concern’ and not imposed on anyone (BG 18.67-69). After the entire teachings of the Gītā had been given, Śrī Kṛṣnạ gives Arjuna the complete choice to decide for himself:
iti te jñānam ākhyātaṃ guhyād guhyataraṃ mayā /
vimṛśyaitad aśeṣeṇa yathecchasi tathā kuru // 18.63
Thus has the wisdom, more secret than all that is secret, been declared to you by Me;
reflect over it all and act as you please.
 Alladi Mahadeva Sastry, trans., The Bhagavad Gītā with the commentary of Sri Shankaracharya, 497.