Near the very end, at 18.64, Kṛṣṇa slows down. Hear again, he says, my supreme word, the most secret of all.
The next verse is that secret. 18.65: fix your mind on me, be my devotee, worship me, bow to me.
But he has said it before. Word for word. At 9.34, the closing verse of chapter 9, the opening of the line is the same in Sanskrit: man-manā bhava mad-bhakto mad-yājī māṁ namaskuru. Chapter 9 is the chapter the tradition names the royal secret.
315 verses separate the two. The greatest secret in the Gītā was not hidden in some final revelation. It was spoken in the middle of the book, and Kṛṣṇa came back to say it again because the first time it was easy to walk past.
The deepest secret was sitting in plain sight.
The verses
BHAGAVAD GĪTĀ · 18.64सर्वगुह्यतमं भूयः श्रृणु मे परमं वचः।इष्टोऽसि मे दृढमिति ततो वक्ष्यामि ते हितम्।।
sarva-guhyatamaṁ bhūyaḥ śṛṇu me paramaṁ vacaḥ
iṣṭo ‘si me dṛḍham iti tato vakṣyāmi te hitam
Hear again My supreme word, the most secret of all; as I am exceedingly fond of you, I am telling you what is good for you.
BHAGAVAD GĪTĀ · 18.65मन्मना भव मद्भक्तो मद्याजी मां नमस्कुरु।मामेवैष्यसि सत्यं ते प्रतिजाने प्रियोऽसि मे।।
man-manā bhava mad-bhakto mad-yājī māṁ namaskuru
mām evaiṣyasi satyaṁ te pratijāne priyo ‘si me
Focus your mind on Me. Be My devotee. Worship Me. Prostrate before Me. You will come to Me alone. I promise you truly; for you are dear to Me.
BHAGAVAD GĪTĀ · 9.34मन्मना भव मद्भक्तो मद्याजी मां नमस्कुरु।
मामेवैष्यसि युक्त्वैवमात्मानं मत्परायणः।।
man-manā bhava mad-bhakto mad-yājī māṁ namaskuru
mām evaiṣyasi yuktvaivam ātmānaṁ mat-parāyaṇaḥ
Focus your mind on Me, be My devotee, worship Me. Bow down to Me. Engaging your mind in this manner and regarding Me as the supreme goal, you will come to Me.
Going deeper
18.64 is a slowing down: hear again my supreme word, the most secret of all, because you are dear to me and I am telling you what is good for you. 18.63 has just said that the knowledge, the mystery of mysteries, has been declared, and that Arjuna should reflect on it and do as he wishes. 18.65 is the secret itself: fix your mind on me, be my devotee, worship me, bow to me, and you will come to me alone. 18.66 follows with the instruction to give up all dharmas. The same opening line stands at 9.34, the last verse of chapter 9, and 9.33 leads into it by saying how much more the devoted sages should worship him. 10.1 opens the next chapter with another supreme word. The story's point is that the secret was said earlier, in the middle of the book. See chapter 9, chapter 18 and what the Gītā says about devotion.