Garbhodakaśāyī Vishnu is an expansion of Mahā Vishnu and with Lord Brahma, resides in a secondary universe that is deep within Maha Vishnu's Brahmanda greater universe.

Garbhodakaśāyī Vishnu below in painting is an expansion of Mahā Vishnu and resides with Lord Brahma deep within Maha Vishnu's Brahmanda greater universe.

There are millions of these Brahmanda universes, each with a Garbhodakasayi Vishnu and Lord Brahma who reside within in a smaller secondary universe that is surrounded by the greater Brahmanda egg shaped universe.

All these Brahmanda's originate from the Body of Maha Vishnu (Kāraṇodakaśāyī Vishnu)

In Gaudīya Vaishnavism the Sātvata-tantra describes three different forms of Vishnu as:

1 - Mahā Vishnu a.k.a. Kāraṇodakaśāyī Vishnu
2 - Garbhodakaśāyī Vishnu Seen in painting below)
3 - Ksirodakaśāyī Vishnu (Paramātmā).

Each form has a different role in the maintenance of the Universe and its inhabitants.

"For material creation, Lord Krishna's plenary expansion assumes three Vishnus.

1 - The first, Mahā Viṣhṇu, creates the total material energy, known as the mahat-tattva.

2 - The second, Garbhodakaśāyī Viṣṇu, enters into all the universes to create diversities.

3 - The third, Kṣīrodakaśāyī Vishnu, is diffused as the all-pervading super soul in all the universes; in the heart of every living being, is known as Paramātmā.

He is present even within the atoms.The real objective of meditation in Yoga is attaining a state of Paramātmā. Anyone who realises them can be liberated from material entanglement."

Garbhodhakaśāyī Viṣṇu is an expansion of Mahā Viṣṇu (expansion of Saṃkarṣaṇa of second caturvyūha, which expands from Nārāyaṇa in Vaikuṇṭhaloka).

Garbhodhakaśāyī Vishnu is realized as the form of Pradyumna within the material universe .

He is the father of Brahmā who appeared from His navel and hence Garbhodakashayi Vishnu is also called Hiraņyagarbha.