No doubt, Brahman is everything. While reciting Bhagwat Geeta, Krishna went in union with Brahman, so whatever he spoke were actually words from Brahman itself.
This is also explained in Anu Gita from Mahabharata Book 14: Aswamedha Parva
Then Arjuna, the son of Pându, having surveyed with delight that
lovely palace, in the company of Krishna, spoke these words: ‘O you of
mighty arms! O you whose mother is Devakî! When the battle was about
to commence, I became aware of your greatness, and that divine form of
yours. But that, O Kesava! which through affection (for me) you
explained before, has all disappeared, O tiger-like man! from my
degenerate mind. Again and again, however, I feel a curiosity about
those topics. But (now), O Mâdhava! you will be going at no distant
date to Dvârakâ.
Krishna, possessed of great glory,replied in these words after
Vâsudeva said: From me, O son of Prithâ! you heard a mystery, and
learnt about the eternal (principle), about piety in (its true) form,
and about all the everlasting worlds. It is excessively disagreeable
to me, that you should not have grasped it through want of
intelligence. And the recollection (of it) now again is not possible
(to me). Really, O son of Pându! you are devoid of faith and of a bad
intellect. And, O Dhanañjaya! it is not possible for me to repeat in
full (what I said before). For that doctrine was perfectly adequate
for understanding the seat, of the Brahman. It is not possible for me
to state it again in full in that way. For then accompanied by my
mystic power, I declared to you the Supreme Brahman.
The Nature is made of 3 gunas, but God manifests most in Satva. Brahman is beyond Prakriti and in Prakriti also. Many think that Prakriti alone has gunas, but actually Prakriti is inert without consciousness/seed/soul from Brahman i.e. Shakti of Prakriti comes from Brahman itself. In a way, Brahman and Shakti are same or more like Father and Mother.
God manifests most in Satva, that is the reason, in ancient times killing of Brahmin(who were Satva dominated), was regarded as biggest sin. Actually the three gunas, arranges themselves into six bands, where the person in white band(in which Satva,dominates over Rajas and Tamas) is at highest level of spirituality and nearest to Brahman. Read in Mahabharat Moksha Dham
The highest Evidence (for all things) says that creatures have six
colours, viz., Dark, Tawny, Blue, Red, Yellow, and White. These
colours proceed from mixtures in various proportions of the three
attributes of Rajas, Tamas, and Sattwa. Where Tamas predominates,
Sattwa falls below the mark, and Rajas keeps to the mark, the result
is the colour called Dark. When Tamas predominates as before, but the
relations between Sattwa and Rajas are reversed, the result is the
colour called Tawny. When Rajas predominates, Sattwa falls below the
mark, and Tamas keeps to the mark, the result is the colour called
Blue. When Rajas predominates as before and the proportion is reversed
between Sattwa and Tamas, the result is the intermediate colour called
Red. That Colour is more agreeable (than the preceding one). When
Sattwa predominates, Rajas falls below the mark and, Tamas keeps to
the mark, the result is the colour called Yellow. It is productive of
happiness. When Sattwa predominates and the proportion is reversed
between Rajas and Tamas, the result is the colour called White. It is
productive of great happiness. The White is the foremost colour. It
is sinless in consequence of its being free from attachment and
aversion. It is without grief, and free from the toil involved in
Pravritti. Hence, White, O prince of Danavas, leads to success (or
Where Satva is highest, that person is definitely the leader of Group like Indra, Garuda etc.,whatever Krishna described. There is another interesting story from Gospel of Ramakrishna Paramhans discussing nature of Brahman.
The three gunas
MASTER: "Yes, it is. But that concept is something far beyond the
ordinary man. Daya springs from sattva. Sattva preserves, rajas
creates, and tamas destroys. But Brahman is beyond the three gunas. It
is beyond Prakriti.
"None of the three gunas can reach Truth; they are like robbers, who
cannot come to a public place for fear of being arrested. Sattva,
rajas, and tamas are like so many robbers.
"Listen to a story. Once a man was going through a forest, when three
robbers fell upon him and robbed him of all his possessions. One of
the robbers said, 'What's the use of keeping this man alive?' So
saying, he was about to kill him with his sword, when the second
robber interrupted him, saying: 'Oh, no! What is the use of killing
him? Tie him hand and foot and leave him here.' The robbers bound his
hands and feet and went away.
After a while the third robber returned and said to the man: 'Ah, I am
sorry. Are you hurt? I will release you from your bonds.' After
setting the man free, the thief said: 'Come with me. I will take you
to the public highway.' After a long time they reached the road. Then
the robber said: 'Follow this road. Over there is your house.' At this
the man said: 'Sir, you have been very good to me. Come with me to my
house ' 'Oh, no!' the robber replied. 'I can't go there. The police
will know it.'
This world itself is the forest. The three robbers prowling here are
sattva, rajas, and tamas. It is they that rob a man of the Knowledge
of Truth. Tamas wants to destroy him. Rajas binds him to the world.
But sattva rescues him from the clutches of rajas and tamas. Under the
protection of sattva, man is rescued from anger, passion, and the
other evil effects of tamas. Further, sattva loosens the bonds of the
world. But sattva also is a robber. It cannot give him the ultimate
Knowledge of Truth, though it shows him the road leading to the
Supreme Abode of God. Setting him on the path, sattva tells him: 'Look
yonder. There is your home.' Even sattva is far away from the
Knowledge of Brahman. Nature of Brahman cannot he described
"What Brahman is cannot be described. Even he who knows It cannot talk
about It. There is a saying that a boat, once reaching the 'black
waters' of the ocean, cannot come back.
Bhagwat Geeta 4.6 describing nature of Brahman briefly
Although I am unborn, the Lord of all living entities, and have an
imperishable nature, yet I appear in this world by virtue of Yogmaya,
my divine power.