In Bhagwat Gita Chapter Ten, Lord Krishna is saying that he is the source of all beings & gods. Not even that as per Lord Krishna, all these universes & creations spring from but a spark of His splendor only. As per Him, He is seated in hearts of all living entities. He Himself is Lord Vishnu, Lord Shiva & Lord Brahma. He Himself is Sun, Moon, Wind & Fire. He Himself is Rama God too. He Himself is Lord of wealth - Kubera. He Himself is Spiritual word Om. He Himself is river Ganges. Among women He Himself is manifested as Goddess of fortune (Lakshmi), Goddess of wisdom (Saraswati) & other Goddesses of fame, memory, intelligence, steadfastness and patience. As per Lord Krishna His opulence is limitless.
Hindus likewise believe in only one God. They glorify and praise His thousands of qualities. For example when God is love, He is symbolically represented by Lord Krishna, God as virtue is represented by lord Rama, God as power is represented by Lord Hanuman, God as wisdom is represented by Goddess Saraswati etc. etc. So these different aspects of Gods represent spirituality, purity, power etc. of the Ultimate One who is endless truth and bliss.
Below are some verses from Bhagwat Gita which explain about various manifestations of One Supreme God.
"aham atma gudakesha
aham adis cha madhyam cha
bhutanam anta eva cha" (Bhagwat Gita: Chapter Ten verse 20)
"Sri Krishna said: I am the Super Soul, O Arjuna, seated in the hearts of all living entities. I am the beginning, the middle and the end of all beings."
"adityanam aham vishnur
jyotisam ravir amsuman
maricir marutam asmi
nakshatranam aham sasi" (Bhagwat Gita: Chapter Ten verse 21)
"Sri Krishna said: Of the Adityas I am Vishnu, of lights I am the radiant Sun, of the Maruts I am Marici, and among the stars I am the Moon."
"rudranam sankaras chasmi
vasunam pavakas chasmi
meruh sikharinam aham" (Bhagwat Gita: Chapter Ten verse 23)
"Sri Krishna said: Of all the Rudras I am Lord Shiva, of the Yakshas and Rakshasas I am the Lord of wealth [Kubera], of the Vasus I am fire [Agni], and of mountains I am Meru."
"pavanah pavatam asmi
ramah shastra-bhrtam aham
jhasanam makaras chasmi
srotasam asmi jahnavi" (Bhagwat Gita: Chapter Ten verse 31)
"Sri Krishna said: Of purifiers I am the Wind, of the wielders of weapons I am Rama, of fishes I am the shark, and of flowing rivers I am the Ganges."
"aksharanam a-karo ’smi
dvandvah samasikasya cha
aham evakshayah kalo
dhataham vishvato-mukhah" (Bhagwat Gita: Chapter Ten verse 33)
"Sri Krishna said: Of letters I am the letter A, and among compound words I am the dual compound. I am also inexhaustible time, and of creators I am Brahma."
"mrityuh sarva-haras chaham
udbhavas cha bhavisyatam
kirtih srir vak cha narinam
smritir medha dhrtih ksama" (Bhagwat Gita: Chapter Ten verse 34)
"Sri Krishna said: I am all-devouring death, and I am the generating principle of all that is yet to be. Among women I am fame, fortune, fine speech, memory, intelligence, steadfastness and patience."
"yac chapi sarva-bhutanam
bijam tad aham arjuna
na tad asti vina yat syan
maya bhutam characharam" (Bhagwat Gita: Chapter Ten verse 39)
"Sri Krishna said: Furthermore, O Arjuna, I am the generating seed of all existences. There is no being—moving or nonmoving—that can exist without Me."
"yad yad vibhutimat sattvam
srimad urjitam eva va
tat tad evavagaccha tvam
mama tejo-’msa-sambhavam" (Bhagwat Gita: Chapter Ten verse 41)
"Sri Krishna said: Know that all opulent, beautiful and glorious creations spring from but a spark of My splendor."
"atha va bahunaitena
kim jnatena tavarjuna
vistabhyaham idam krtsnam
ekamsena sthito jagat" (Bhagwat Gita: Chapter Ten verse 42)
"Sri Krishna said: But what need is there, Arjuna, for all this detailed knowledge? With a single fragment of Myself I pervade and support this entire universe."