In Gita, I could find few relative references.
I personally believe that "BrahmA", "Vishnu", "Shiva" all are the name of the same supreme reality, which is in oneness.
BG 13.14 - That (Knowable), which has hands and feet everywhere, which has eyes, heads and mouths everywhere, which has ears everywhere, exists in creatures by pervading them all.
BG 11.15 - ... Brahma the ruler, sitting on a lotus seat,
BG 5.10 - One who acts by dedicating actions to Brahman and by renouncing attachment, he does not become polluted by sin, just as a lotus leaf is not by water.
Hence, it's typically depicted with several heads, hands, legs, eyes & sitting on lotus unaffected from the Prakruti.
There is a namesake Vishnu reference, which refers to the sons of Aditi:
BG 10.21 - Among the Adityas [viz Dhata, Mitra, Aryama, Rudra, Varuna, Surya, Bhaga, Vivasvan, Pusa, Savita, Tvasta and Visnu.-Tr.] I am Vishnu; ... -- translation Gambhirananda
Few verses in chapter-11 depicts the famous all deity form:
BG 11.10 - Having many faces and eyes, possessing many wonderful sights, adorned with numerous celestial ornaments, holding many uplifted heavenly weapons;
BG 11.15 - Arjuna said O God, I see in Your body all the gods as also hosts of (various) classes of beings; Brahma the ruler, sitting on a lotus seat, and all the heavenly sages and serpents.
BG 11.16 - I see You as possessed of numerous arms, bellies, mouths and eyes; as having infinite forms all around. O Lord of the Universe, O Cosmic Person, I see not Your limit nor the middle, nor again the beginning!
BG 11.17 - I see You as wearing a diadem, wielding a mace, and holding a disc; a mass of brilliance glowing all around, difficult to look at from all sides, possessed of the radiance of the blazing fire and sun, and immeasurable.
BG 11.22 Those who are the Rudras, the Adityas, the Vasus and the Sadhyas, the Visve (-devas), the two Asvins, the Maruts and the Usmapas, and hosts of Gandharvas, Yaksas, demons and Siddhas-all of those very ones gaze at You, being indeed struck with wonder.
BG 11.23 - O mighty-armed One, seeing Your immense form with many mouths and eyes, having numerous arms, thighs and feet, with many bellies, and fearful with many teeth, the creatures are struck with terror, and so am I.
Another famous depiction of Vishnu is sleeping on Adi Shesha is discussed here.
Another form depicts as of whole universe silently pervades as the Vishnu:
BG 9.4 - This whole universe is pervaded by Me in My unmanifested form. All beings exist in Me, but I am not contained in them!
"Shankara" is directly referred in Gita. Some people [mistakenly] consider "Shiva = Shankara":
BG 10.23 - Among the Rudras [Aja, Ekapada, Ahirbudhnya, Pinaki, Aparajita, Tryam-baka, Mahesvara, Vrsakapi, Sambhu, Harana and Isvara] I am Shankara, ... -- translation Gambhirananda
However, this is not the Rudra Shankara. But Shiva, which is the nirguna Brahman. Usually Shiva is depicted in the form of linga.
The linga (phallus) plants the seeds as a father and the yoni (womb), which accepts them as a mother to form the universe.
BG 14.3 - My womb is the great-sustainer. In that I place the seed. From that, O scion of the Bharata dynasty, occurs the birth of all things.
BG 14.4 - O son of Kunti, whatever forms are born from all the wombs, of them the great-sustainer is the womb; I am the father who deposits the seed.