Despite denying earlier reports of Sejeong and Mina debuting in a 3-member girl group in June, Jellyfish Entertainment confirmed on June 7 that the two I.O.I members would debut within the month in what would be the company's first girl group.
Because of YMC Entertainment's special terms in which I.O.I's members are allowed to do activities under their respective companies while I.O.I was on break or while the group promoted in sub-units, there were no complications with the plan.

Nayoung was also confirmed as a member on June 10 and on June 13, Jellyfish Entertainment revealed that Gugudan would be a nine-member group.
On June 17, Jellyfish Entertainment announced the group name.

On June 22, the group confirmed that they were going for a "mermaid" concept.
Following the announcement, a highlight medley of the group's debut mini-album was released on June 24.
They debuted on June 28, 2016 with the mini-album, Act.
1 The Little Mermaid, with "Wonderland" as the title song.
Their debut showcase was held at the Yes24 Live Hall on the same day as the album's release.

Gugudan participated in Jellyfish Entertainment's annual winter project, Jelly Christmas 2016, with their label mates Seo In-guk, VIXX, Park Yoon-ha, Park Jung-ah, Kim Gyu-sun, Kim Ye-won and Jiyul.
The title track, "Falling (니가 내려와)" was released digitally on December 13, 2016.
Despite denying earlier reports of Sejeong and Mina debuting in a 3-member girl group in June, Jellyfish Entertainment confirmed on June 7 that the two I.O.I members would debut within the month in what would be the company's first girl group. Because of YMC Entertainment's special terms in which I.O.I's members are allowed to do activities under their respective companies while I.O.I was on break or while the group promoted in sub-units, there were no complications with the plan.
Nayoung was also confirmed as a member on June 10 and on June 13, Jellyfish Entertainment revealed that Gugudan would be a nine-member group. On June 17, Jellyfish Entertainment announced the group name.
On June 22, the group confirmed that they were going for a "mermaid" concept. Following the announcement, a highlight medley of the group's debut mini-album was released on June 24. They debuted on June 28, 2016 with the mini-album, Act. 1 The Little Mermaid, with "Wonderland" as the title song. Their debut showcase was held at the Yes24 Live Hall on the same day as the album's release.
Gugudan participated in Jellyfish Entertainment's annual winter project, Jelly Christmas 2016, with their label mates Seo In-guk, VIXX, Park Yoon-ha, Park Jung-ah, Kim Gyu-sun, Kim Ye-won and Jiyul. The title track, "Falling (니가 내려와)" was released digitally on December 13, 2016.
Gugudan