Krishna turns to Arjuna and says: because you do not find fault in me, I will share the deepest secret with you. This is not book-knowledge alone. It comes with vijnan — direct inner experience. And knowing it, you will be free from all that binds you.
Think of an elder in the family who has lived through everything — joys, losses, hard-won wisdom. That elder does not share the real lessons with just anyone. Only the one who listens without suspicion, without nitpicking, receives the full truth. Krishna sees that quality in Arjuna.
The word 'guhyatamam' means the most secret of secrets. Not secret because it is hidden, but because it requires a certain readiness of heart. Without that openness, even the finest teaching passes right through.