B.G 11.31

आख्याहि मे को भवान् उग्ररूपो नमोऽस्तु ते देववर प्रसीद। विज्ञातुमिच्छामि भवन्तमाद्यं न हि प्रजानामि तव प्रवृत्तिम् ॥३१॥

ākhyāhi me ko bhavān ugrarūpo namo'stu te devavara prasīda। vijñātum icchāmi bhavantam ādyaṁ na hi prajānāmi tava pravṛttim ॥31॥

[आख्याहि (ākhyāhi) - declare; मे (me) - to me; कः (kaḥ) - who; भवान् (bhavān) - you; उग्ररूपः (ugrarūpaḥ) - of fierce form; नमः (namaḥ) - salutations; अस्तु (astu) - let there be; ते (te) - to you; देववर (devavara) - O best among gods; प्रसीद (prasīda) - be gracious; विज्ञातुम् (vijñātum) - to know; इच्छामि (icchāmi) - I wish; भवन्तम् (bhavantam) - you; आद्यम् (ādyaṁ) - the primordial one; न (na) - not; हि (hi) - indeed; प्रजानामि (prajānāmi) - do I know; तव (tava) - your; प्रवृत्तिम् (pravṛttim) - origin or nature;]

Declare to me — who are you, O fierce one? Salutations to you, O best of gods, be gracious. I wish to know you as the primordial being, for I do not indeed know your origin or nature.

Gīta Bhāshya 11.31

The question “Who are you?” in the context is asked to know various other aspects of the supreme being. It does not mean Arjuna is completely ignorant of the Lord's significance, as he is addressed as 'O Viṣṇu' (in verse 30).

धर्मान्तरज्ञानार्थमेव "को भवान्" इति पृच्छति। यथा कश्चित् किञ्चित् नामादिकं जानन्नपि जातिज्ञानार्थं पृच्छति "कस्त्वं" इति। यदि तमेव न जानाति तर्हि "विष्णो" इत्येव सम्बोधना न स्यात्। "त्वमक्षरम्" इत्यादि च॥३१॥

dharmāntarajñānārtham eva "ko bhavān" iti pṛcchati। yathā kaścit kiñcit nāmādikaṁ jānann api jātijñānārthaṁ pṛcchati "kastvaṁ" iti। yadi tam eva na jānāti tarhi "viṣṇo" ity eva sambodhanā na syāt। "tvam akṣaram" ityādi ca॥31॥

[धर्मान्तरज्ञानार्थम् (dharmāntarajñānārtham) - for the purpose of knowing another aspect of the nature; एव (eva) - only; "को भवान्" (ko bhavān) - "who are you"; इति (iti) - thus; पृच्छति (pṛcchati) - he asks; यथा (yathā) - just as; कश्चित् (kaścit) - someone; किञ्चित् (kiñcit) - something; नामादिकम् (nāmādikam) - name etc.; जानन् (jānann) - knowing; अपि (api) - even; जातिज्ञानार्थम् (jātijñānārtham) - for the purpose of knowing the species or class; पृच्छति (pṛcchati) - asks; "कस्त्वम्" (kastvaṁ) - "who are you"; इति (iti) - thus; यदि (yadi) - if; तम् एव (tam eva) - him only; न (na) - not; जानाति (jānāti) - he knows; तर्हि (tarhi) - then; "विष्णो" (viṣṇo) - O Viṣṇu; इति एव (iti eva) - just thus; सम्बोधनाः (sambodhanāḥ) - address; न (na) - not; स्यात् (syāt) - would be; "त्वम् अक्षरम्" (tvam akṣaram) - "you are the imperishable"; इत्यादि (ityādi) - and so on; च (ca) - also;]

He asks “Who are you?” only for the sake of knowing another aspect of the nature. Just as someone, even knowing a name or the like, may ask “Who are you?” to know the class or nature. If he did not know him at all, then the address “O Viṣṇu” would not apply. And likewise the statement “You are the imperishable” and so on.