The ancedote of Gundicha devi and Jaggannath blessing her is a Lokachar katha. Thus, I can't exactly provide the referances. Maybe it might be mentioned in Sthala Puranas of Puri.
Still the boon given to Indradyumna is as follows-
Skanda Puran, 2nd Skanda- Puruṣottama-kṣetra-māhātmya, Chapter 29 - The Lord Grants Boons to Indradyumna
- O lord of the earth, you should celebrate ‘the great procession named Guṇḍicā. The very glorification of this liberates a man from sins.
- The fifth day of the lunar month of Māgha and the eighth day in the bright half of Caitra—these are the best days for the great festival named Guṇḍicā.
- Particularly the second day in the month of Āṣāḍha in conjunction with Puṣya constellation is conducive to salvation. The festival shall be celebrated on the specified lunar day even if the constellation does not coincide. It is pleasing to me always.
- On the second day, in the bright half of the lunar month of Āṣāḍha, in conjunction with Puṣya constellation, the devotees should place Rāma and me along with Bhadrā on the chariot.
- For the performance of the great festival, many Brāhmaṇas should be propitiated. The Guṇḍicā pavilion should be where I was born before.
(It was Gundicha temple where Jagannath and the siblings were carved out by Vishwakarma from Wood, thus Shree Krishn called it as his birth place.)
- Due to his request and your devotion I shall stay here permanently. I shall go (there) for nine days. Then I will come here.
So, coming to the tale of Gundicha queen, What is the relation between Lord Jagannath and Goddess Gundicha?
As per Lokamath, Gundicha was the Queen of King Indradyumna, who started the original Shri Jagannath Temple at Puri. After the temple was set, the Lord blessed the King and the Queen and offered boons to them if required. The King said he should remain childless so that his successors would not claim credit for having constructed the Temple.
Till date Lord Jagannath makes offerings to King Indradyumna and Queen Gundicha on their death anniversary as there was no successor of the King and the Queen.
Thus the relationship between Lord Jagannath and Queen Gundicha is like that of mother and son.
Another version states that Gundicha was childless and Shree Krishn said that, he was pleased by her selfless love and devotion thus, he blessed her that he would every year come to Gundicha temple like a child to get her all love , leaving his all duties of being Jaggannath (the lord of universe). He called her as maternal aunt (Mother's sister/mausi) because he said, Mausi loves kids same way as a mothers do.
(Aslo to state there is no temple of Yashoda or Devaki in Puri temple, so he would definetly miss his mother.)
It is believed that their aunt gives a welcome to the three siblings with a special delicacy made of rice called 'padoapitha'.
Also, while leaving back for Shri mandir, after rath yatra, the Rathas are brought near the temple of Gundicha devi knw as Mausi ghar. And they depart from there.
Now there's a confusion to who actually Gundicha is. As there is a form of Durga too called Gundicha (the goddess of smallpox). In oria Gundi means Smallbox. Thus she is too called the Mausi of Jaggannath.
I think both are same, yet can't affirm it.
कालिकार्पणमस्तु । 🌺