The whole Gītā is a report. Sañjaya, the charioteer of the blind king Dhṛtarāṣṭra, sees the battlefield from far away and tells the king what is said.
Kṛṣṇa speaks his last words at 18.72. Arjuna answers at 18.73. Then the voice returns to Sañjaya for the final five verses.
The very last verse, 18.78, is not a teaching. It is a verdict: wherever there is Kṛṣṇa, the lord of yoga, and Arjuna the archer, there is fortune, victory and wealth. It closes on matir mama: this is my firm conviction.
After 700 verses, the last word goes to the one who listened. And he believes.
Going deeper
The last verses of the Gītā belong to the narrator. 18.72 is Kṛṣṇa's last question and 18.73 is Arjuna's answer. 18.74 hands the voice to Sañjaya, who says that hearing the dialogue made his hair stand on end. 18.75 credits Vyāsa's grace for his hearing, 18.76 says he rejoices again and again as he remembers it, and 18.77 says he is amazed as he remembers the marvellous form of Hari. 18.78 closes the book: wherever there is Kṛṣṇa, the Lord of yoga, and Arjuna the archer, there is always fortune, victory, wealth and sound morality, and this is his firm conviction. So the last statement in the Gītā is not a command from Kṛṣṇa but a verdict from the one who listened. The close is in chapter 18, and what the Gītā says about dharma is tied to this verse.