Garbhodakaśāyī Vishnu is an expansion of Vishnu. In Gaudīya Vaishnavism the Sātvata-tantra describes three different forms of Vishnu as: Mahā Vishnu, Garbhodakaśāyī Vishnu and 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. The first, Mahā Viṣhṇu, creates the total material energy, known as the mahat-tattva. The second, Garbhodakaśāyī Viṣṇu, enters into all the universes to create diversities and 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."
Garbhodakaśāyī Viṣṇu is an expansion or overload of Mahā Viṣṇu (expansion of Saṃkarṣaṇa of second caturvyūha, which expands from Nārāyaṇa in Vaikuṇṭhaloka). The latter is realized as the expansion of Pradyumna.
- Quoted from the Sātvata-tantra translation by A. C. Bhaktivedanta Swami Prabhupāda