In the 13th Chapter of the BG we have a set of qualifications the liberation-seeker has to develop, nurture, so as to gain the knowledge of Nirguna Brahman which is a sine qua non for liberation. Among these, the Lord says is:
मयि चानन्ययोगेन भक्तिरव्यभिचारिणी ।
विविक्तदेशसेवित्वमरतिर्जनसंसदि ॥ १० ॥ 13.10
And unwavering devotion to Me with single-minded concentration; inclination to repair into a clean place; lack of delight in a crowd of people;
Shankara comments on that:
,मयि च इश्वरे अनन्ययोगेन अपृथक्समाधिना न अन्यो भगवतो वासुदेवात् परः अस्ति? अतः स एव नः गतिः इत्येवं निश्चिता अव्यभिचारिणी बुद्धिः अनन्ययोगः? तेन भजनं भक्तिः न व्यभिचरणशीला अव्यभिचारिणी। सा च ज्ञानम्।
Ca, and; avyabhicarini, unwavering-not having any tendency to deviate; bhaktih, devotion; mayi, to Me, to God; ananya-yogena, with single-minded concentration, with undivided concentration-ananyayogah is the decisive, unswerving conviction of this kind: 'There is none superior to Lord Vasudeva, and hence He alone is our Goal'; adoration with that. That too is Knowledge.
Having said that the Lord proceeds to teach the Knowable, Brahman, for liberating knowledge:
ज्ञेयं यत्तत्प्रवक्ष्यामि यज्ज्ञात्वाऽमृतमश्नुते।
अनादिमत्परं ब्रह्म न सत्तन्नासदुच्यते।।13.12।।
I shall speak of that which is to be known, by realizing which one attains Immortality. The supreme Brahman is without any beginning. That is called neither being nor non-being.
That is Nirguna Brahman, going by the epithet: That is called neither being nor non-being.
Thus, the devotion to the Saguna Brahman, where difference between the devotee and the Lord is articulated and maintained, and grace prayed for. This is an essential requirement for the realization of the NB where no difference is possible between the seeker and the sought.
One can see that what Shankara has said in BSB 2.3.41 is in no way wanting in the need for the grace of Paramatma:
परस्मादात्मनः कर्माध्यक्षात्सर्वभूताधिवासात्साक्षिणश्चेतयितुरीश्वरात्तदनुज्ञया कर्तृत्वभोक्तृत्वलक्षणस्य संसारस्य सिद्धिः ; तदनुग्रहहेतुकेनैव च विज्ञानेन मोक्षसिद्धिर्भवितुमर्हति । कुतः ? तच्छ्रुतेः |
This means: It is only by the grace of the Lord, Ishwara, that one secures the Knowledge that results in liberation.
For both samsara siddhi and moksha siddhi, the indispensable role of Brahman is emphasized by Shankara on the basis of the shruti.
In the BGB too Shankara has said a similar idea while introducing the 15 Chapter:
यस्मात् मदधीनं कर्मिणां कर्मफलं ज्ञानिनां च ज्ञानफलम्, अतः भक्तियोगेन मां ये सेवंते ते मम प्रसादात् ज्ञानप्राप्तिक्रमेण गुणातीताः मोक्षं गच्छन्ति ।
Karma phala is abhyudaya or jnana praapti yogyataa and jnana phala is moksha.
Thus SB and NB are so intimately relevant in the Advaitic Sadhana.
warm regards
subbu