Garbhodakaśāyī Vishnu below in painting is an expansion of Mahā Vishnu a.k.a. Kāraṇodakaśāyī Vishnu

Garbhodakaśāyī Vishnu below in painting is an expansion of Mahā 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.

The rare appearance of Lord Caitanya Mahaprabhu explained -

Harinam Sankirtan is the greatest gift given to this world!

Harinam Sankirtan is Lord Caitanya Mahaprabhu’s great gift to this particular Kali-yuga and it’s very rare because Lord Caitanya does not come to every Kali-yuga.

He only comes to just "one" Kali-yuga out of every 1000 which is once in a day/night of Brahma or once every 8 billion 640 million human years.

There are 1000 Kali-yuga’s in one-half day (day-time only) of Lord Brahmas time (Sunrise to Sunset)

It takes 4 billion 320 million human years for 1000 Maha-yugas to pass from Brahma's day from Sunrise to Sunset.

There are no Maha-yugas (also known as a Divya-yugas) during Brahma's night (Sunset to Sunrise).

A Maha-yuga is a cycle of

Satya,
Treta,
Dvāpara,
Kali-yuga’s.

They all happens, as said above, during Lord Brahma’s Sunrise to Sunset day-time period taking 4 billion 320 million earth or human years.

Then add the same duration of time (4 billion 320 million earth or human years) for Brahma’s inactive night from sunset to sunrise making a 24 hour day 8 billion 640 million human years.

So we must understand that Lord Caitanya's appearance in just one Kali-yuga, (the same Kali-yuga we are in now), is very, very rare.

This means Lord Caitanya does not appear in the other 999 Kali-yugas during Brahma’s day time period from sunrise to sunset.

The time of Brahma's night between sunset and sunrise is when this material universe experiences a partial annihilation. This happens over a period of 4 billion 320 million human years which is Brahma's night.

So one "day-night 24 hour period" of Brahma equals 8 billion 640 million human years as already explained.

Lord Caitanya therefore comes “once” in a 24 hour day of Lord Brahma at the beginning of a Kali-yuga.

And so does Lord Krishna in His original Vrindavana  Form, who also  comes "once" in a 24 hour day of Brahma at the end of a Dvāpara-yuga.