This 28-verses chapter deals with an un-wavering devotion to the Supreme to attain Him besides the science of meditation to realize the Brahman towards the same end. It can be seen below how v5 places the cart before the horse, and why v9-v14 too, are interpolations that is going by their content that's out of context.
However, the interpolations that take the cake are the superstitious v23-v27 that pollute an otherwise thought-elevating work, and as already noted, it was to Sir Edwin Arnold credit that he deleted these from his timeless The Song Celestial.
So, to see all these interpolations in their improper place, one may appreciate the text and the context of this chapter that opens with Arjuna's query to Krishna thus:
Ch8, V1
O Lord appraise what's Brahman
Lies what within 'n backs action
Nature of deities besides the beings.
kiṁ tad brahma kim adhyātmaṁ kiṁ karma puruṣhottama
adhibhūtaṁ cha kiṁ proktam adhidaivaṁ kim uchyate
Ch8, V2
What is that guides bodily acts,
What makes yogis realize Thee?
adhiyajñaḥ kathaṁ ko 'tra dehe 'smin madhusūdana
prayāṇa-kāle cha kathaṁ jñeyo 'si niyatātmabhiḥ
And thus spoke Krishna,
Ch8, V3
Self-Imperishable is Brahman
But dwells it yet there in beings
Brings that forth is Act Supreme.
akṣharaṁ brahma paramaṁ svabhāvo 'dhyātmam uchyate
bhūta-bhāvodbhava-karo visargaḥ karma-sanjñitaḥ
Ch8, V4
Perish as beings all in time
Spirit that lasts of them is Me.
adhibhūtaṁ kṣharo bhāvaḥ puruṣhaśh chādhidaivatam
adhiyajño 'ham evātra dehe deha-bhṛitāṁ vara
Now note this -
V5
anta-kāle cha mām eva smaran muktvā kalevaram
yaḥ prayāti sa mad-bhāvaṁ yāti nāstyatra sanśhayaḥ
Those who relinquish the body while remembering Me at the moment of death will come to Me. There is certainly no doubt about this.
And going by the following, the above is a prehash of the same -
Ch8, V6
In the end the way one tends
Charts that future course he takes.
yaṁ yaṁ vāpi smaran bhāvaṁ tyajatyante kalevaram
taṁ tam evaiti kaunteya sadā tad-bhāva-bhāvitaḥ
Ch8, V7
If thou act with this in mind
In the end thou gain Me true,
By My word now opt for war
With thy strength 'n skill I gave.
tasmāt sarveṣhu kāleṣhu mām anusmara yudhya cha
mayyarpita-mano-buddhir mām evaiṣhyasyasanśhayam
Ch8, V8
Me they reach whoso keep
On Me focus as they work.
yam yam vapi smaran bhavam tyajatyante kalevaram
tam tam evaiti kaunteya sada tad-bhava-bhavitah
So, as v5, places the v8 cart before the horse, here comes v3's expansion of after the closure of the subject as above
V9
kaviṁ purāṇam anuśhāsitāram
aṇor aṇīyānsam anusmared yaḥ
sarvasya dhātāram achintya-rūpam
āditya-varṇaṁ tamasaḥ parastāt
He who is omniscient, timeless, the commander, subtler than the subtlest, protector of all, incomprehensible, brilliant like the sun, beyond darkness, (one) contemplates (him).
Now follows a mixture of lectures on yoga, meditation, celibacy etc.
V10
prayāṇa-kāle manasāchalena
bhaktyā yukto yoga-balena chaiva
bhruvor madhye prāṇam āveśhya samyak
sa taṁ paraṁ puruṣham upaiti divyam
At the time of departure, endowed with devotion, an unwavering mind, as well as the power of yoga, fully establishing the praanaas in the centre of the eyebrows, he attains that supreme divine person.
V11
yad akṣharaṁ veda-vido vadanti
viśhanti yad yatayo vīta-rāgāḥ
yad ichchhanto brahmacharyaṁ charanti
tat te padaṁ saṅgraheṇa pravakṣhye
Scholars of the Vedas describe Him as Imperishable; great ascetics practice the vow of
celibacy and renounce worldly pleasures to enter into Him. I shall now explain to you
briefly the path to that goal.
V12
arva-dvārāṇi sanyamya mano hṛidi nirudhya cha
mūrdhnyādhāyātmanaḥ prāṇam āsthito yoga-dhāraṇām
Restraining all the gates of the body and fixing the mind in the heart region, and then drawing the life-breath to the head, one should get established in steadfast yogic concentration.
V13
oṁ ityekākṣharaṁ brahma vyāharan mām anusmaran
yaḥ prayāti tyajan dehaṁ sa yāti paramāṁ gatim
One who departs from the body while remembering Me, the Supreme Personality, and chanting the syllable Om, will attain the supreme goal.
V14
ananya-chetāḥ satataṁ yo māṁ smarati nityaśhaḥ
tasyāhaṁ sulabhaḥ pārtha nitya-yuktasya yoginaḥ
O Parth, for those yogis who always think of Me with exclusive devotion, I am easily attainable because of their constant absorption in Me.
And here we have the v23-v27 superstitious kit, the pièce de résistance that was broached at the beginning of this chapter introduction, and before, which, if literally taken, would imply that if one dies when the moon is on the ascent he would be heaven bound and, to hell, if it's other way round. So, Sir Edwin Arnold, one of the foremost to translate the Gita that was way back in 1885, dismissed these as the work of some vedānti and thought it fit, justifiably at that, not to include them in his The Song Celestial, of course, along with the tailpiece v28.
In this connection it may be noted that the relationship between the state, in which a person dies, and his imminent rebirth is covered in Ch14, v14 'n v15, which are seemingly authentic.
V23
yatra kāle tvanāvṛittim āvṛittiṁ chaiva yoginaḥ
prayātā yānti taṁ kālaṁ vakṣhyāmi bharatarṣhabha
But, (there exists) the path of no return for a yogi who is leaving his body, and also the path of return, I shall speak about those, O scion of the Bharatas.
V24
agnir jyotir ahaḥ śhuklaḥ ṣhaṇ-māsā uttarāyaṇam
tatra prayātā gachchhanti brahma brahma-vido janāḥ
Fire, light, day, the bright (fortnight of the month), the northern movement comprising six months; those people who have departed through that path, knowers of brahman attain brahman.
V25
dhūmo rātris tathā kṛiṣhṇaḥ ṣhaṇ-māsā dakṣhiṇāyanam
tatra chāndramasaṁ jyotir yogī prāpya nivartate
Smoke, night, darkness and the southern movement comprising six months; the yogi (travels through) that path, attains the light of the moon, to return.
V26
śhukla-kṛiṣhṇe gatī hyete jagataḥ śhāśhvate mate
ekayā yātyanāvṛittim anyayāvartate punaḥ
For, bright and dark, both these paths have been known since eternity. By one, the traveller does not have to return, by the other, he has to return again.
V27
naite sṛitī pārtha jānan yogī muhyati kaśhchana
tasmāt sarveṣhu kāleṣhu yoga-yukto bhavārjuna
Yogis who know the secret of these two paths, O Parth, are never bewildered. Therefore, at all times be situated in Yog (union with God).
V28
vedeṣhu yajñeṣhu tapaḥsu chaiva
dāneṣhu yat puṇya-phalaṁ pradiṣhṭam
atyeti tat sarvam idaṁ viditvā
yogī paraṁ sthānam upaiti chādyam
The yogis, who know this secret, gain merit far beyond the fruits of Vedic rituals, the study of the Vedas, performance of sacrifices, austerities, and charities. Such yogis reach the Supreme Abode.
It is worth noting that v1- v4, v6- v8 and v15-v22 of this chapter, if read together would bear an unmistakable continuity of argument that these thirteen interpolations as above deprive it.