Arjuna sees the celestial serpent upon which the Garbhodakasayi Visnu lies in the lower regions of the universe. This snake bed is called Vasuki. There are also other snakes known as Vasuki. BG 1972 purports

Expressions researched:
"the celestial serpent upon which the Garbhodakaśāyī Viṣṇu lies in the lower regions of the universe. This snake bed is called Vāsuki. There are also other snakes known as Vāsuki"

Bhagavad-gita As it is

BG Chapters 7 - 12

Arjuna sees everything in the universe; therefore he sees Brahmā, who is the first creature in the universe, and the celestial serpent upon which the Garbhodakaśāyī Viṣṇu lies in the lower regions of the universe. This snake bed is called Vāsuki. There are also other snakes known as Vāsuki. Arjuna can see from the Garbhodakaśāyī Viṣṇu up to the topmost part of the universe on the lotus-flower planet where Brahmā, the first creature of the universe, resides.

BG 11.15 (1972), Translation and Purport

Arjuna said: My dear Lord Kṛṣṇa, I see assembled together in Your body all the demigods and various other living entities. I see Brahmā sitting on the lotus flower as well as Lord Śiva and many sages and divine serpents.

Arjuna sees everything in the universe; therefore he sees Brahmā, who is the first creature in the universe, and the celestial serpent upon which the Garbhodakaśāyī Viṣṇu lies in the lower regions of the universe. This snake bed is called Vāsuki. There are also other snakes known as Vāsuki. Arjuna can see from the Garbhodakaśāyī Viṣṇu up to the topmost part of the universe on the lotus-flower planet where Brahmā, the first creature of the universe, resides. That means that from the beginning to the end, everything could be seen by Arjuna sitting in one place on his chariot. This was possible by the grace of the Supreme Lord, Kṛṣṇa.