avyakto ’kṣhara ityuktas tam āhuḥ paramāṁ gatim
yaṁ prāpya na nivartante tad dhāma paramaṁ mama||21|| 

Meaning : The invisible and never-dying reality is called the highest goal. Those who reach it never come back. That place is My eternal home.

paras tasmāt tu bhāvo ’nyo ’vyakto ’vyaktāt sanātanaḥ
yaḥ sa sarveṣhu bhūteṣhu naśhyatsu na vinaśhyati||20|| 

Meaning : But beyond this unmanifest, there is another eternal, unmanifest reality. Even when all beings are destroyed, that higher reality does not perish.

bhūta-grāmaḥ sa evāyaṁ bhūtvā bhūtvā pralīyate
rātryāgame ’vaśhaḥ pārtha prabhavatyahar-āgame||19|| 

Meaning : This group of living beings (bhūta-grāma) keeps taking birth again and again, and then dissolves at the arrival of Brahma’s night. Powerless, O Arjuna, they are born again at the coming of his day.

avyaktād vyaktayaḥ sarvāḥ prabhavantyahar-āgame
rātryāgame pralīyante tatraivāvyakta-sanjñake||18|| 

Meaning : At the beginning of Brahma’s day, all living beings come into existence from the unmanifest. At the coming of night, they dissolve back into the unmanifest again.

sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ
rātriṁ yuga-sahasrāntāṁ te ’ho-rātra-vido janāḥ||17|| 

Meaning : People who understand the truth know that one day of Brahma lasts for a thousand yugas, and his night also lasts for a thousand yugas. A day of Brahma (the creator) lasts for 1,000 yugas, and his night is equally long. Those who understand this know the vast scale of cosmic time.1,000 yugas = 1 day of Brahma = approx. 4.32 billion years
So 1 full day and night = 8.64 billion years

ā-brahma-bhuvanāl lokāḥ punar āvartino ’rjuna
mām upetya tu kaunteya punar janma na vidyate||16|| 

Meaning : O Arjuna, even the worlds up to Brahma’s abode are places of repeated birth and death. But those who reach Me never take birth again.

mām upetya punar janma duḥkhālayam aśhāśhvatam
nāpnuvanti mahātmānaḥ sansiddhiṁ paramāṁ gatāḥ||15|| 

Meaning : Great souls who reach Me do not take birth again in this world, which is full of suffering and temporary. They have attained the highest perfection.

ananya-chetāḥ satataṁ yo māṁ smarati nityaśhaḥ
tasyāhaṁ sulabhaḥ pārtha nitya-yuktasya yoginaḥ||14|| 

Meaning : O Arjuna (Pārtha), for the yogi who always thinks of Me with an undivided mind, constantly remembering Me without deviation — I am easily attainable for such a steadfast devotee.

oṁ ityekākṣharaṁ brahma vyāharan mām anusmaran
yaḥ prayāti tyajan dehaṁ sa yāti paramāṁ gatim||13|| 

Meaning : Whoever, at the time of death, chants the one-syllabled “Om,” while remembering Me (Krishna), and leaves the body, that person attains the supreme goal.

sarva-dvārāṇi sanyamya mano hṛidi nirudhya cha
mūrdhnyādhāyātmanaḥ prāṇam āsthito yoga-dhāraṇām||12|| 

Meaning : Having controlled all the gates of the body (senses), and restrained the mind in the heart, placing the life-force (prāṇa) at the top of the head, one becomes firmly established in yogic concentration.