Which verses in Bhagavad-Gita Support Advaitic Doctrine ?
There are at least 3 verses that have strong Advaita flavor.
As a kindled fire reduces all fuel to ashes, so, Arjuna! does the fire of knowledge reduce all works to ashes.
All these (four) are noble; but the knower, I deem to be My very Self; for, with an integrated self, he has resorted to Me alone, the goal without parallel.
Moved by compassion for those (devotees) and dwelling in their mental states, with the lustrous flame of knowledge, I destroy the darkness of ignorance.
BG 6.30: For those who see me everywhere and see all things in me, I am never lost, nor are they ever lost to me.
BG 9.7-8: At the end of one kalp, all living beings merge into My primordial material energy. At the beginning of the next creation, O son of Kunti, I manifest them again. Presiding over My material energy, I generate these myriad forms again and again, in accordance with the force of their natures.
BG 9.4: This entire cosmic manifestation is pervaded by Me in My unmanifest form. All living beings dwell in Me, but I do not dwell in them
BG 9.10: Working under My direction, this material energy brings into being all animate and inanimate forms, O son of Kunti. For this reason, the material world undergoes the changes (of creation, maintenance, and dissolution).
BG 9.15: Others, engaging in the yajña of cultivating knowledge, worship Me by many methods. Some see Me as undifferentiated oneness that is non-different from them, while others see Me as separate from them. Still others worship Me in the infinite manifestations of My cosmic form.
BG 9.18: I am the Supreme Goal of all living beings, and I am also their Sustainer, Master, Witness, Abode, Shelter, and Friend. I am the Origin, End, and Resting Place of creation; I am the Storehouse and Eternal Seed.