After the expiry of the elephant's life (as Gajendra), he (Indradyumna) was promoted to Vaikunthaloka to become a personal associate of the Supreme Personality of Godhead, Narayana, in a body exactly like that of the Lord. This is called sarupya-mukti

Expressions researched:
"Immediately after the expiry of the elephant's life, he was promoted to Vaikuṇṭhaloka to become a personal associate of the Supreme Personality of Godhead, Nārāyaṇa, in a body exactly like that of the Lord. This is called sārūpya-mukti"

SB Canto 8

Although Agastya Muni cursed Mahārāja Indradyumna to become an elephant, the curse was indirectly a benediction, for by undergoing one life as an elephant, Indradyumna Mahārāja ended the reactions for all the sins of his previous life. Immediately after the expiry of the elephant's life, he was promoted to Vaikuṇṭhaloka to become a personal associate of the Supreme Personality of Godhead, Nārāyaṇa, in a body exactly like that of the Lord. This is called sārūpya-mukti.

SB 8.4.10, Translation and Purport:

Agastya Muni then spoke this curse against the King: This King Indradyumna is not at all gentle. Being low and uneducated, he has insulted a brāhmaṇa. May he therefore enter the region of darkness and receive the dull, dumb body of an elephant.

An elephant is very strong, it has a very big body, and it can work very hard and eat a large quantity of food, but its intelligence is not at all commensurate with its size and strength. Thus in spite of so much bodily strength, the elephant works as a menial servant for a human being. Agastya Muni thought it wise to curse the King to become an elephant because the powerful King did not receive Agastya Muni as one is obliged to receive a brāhmaṇa. Yet although Agastya Muni cursed Mahārāja Indradyumna to become an elephant, the curse was indirectly a benediction, for by undergoing one life as an elephant, Indradyumna Mahārāja ended the reactions for all the sins of his previous life. Immediately after the expiry of the elephant's life, he was promoted to Vaikuṇṭhaloka to become a personal associate of the Supreme Personality of Godhead, Nārāyaṇa, in a body exactly like that of the Lord. This is called sārūpya-mukti.