It is unfortunately only a mini-film, like M7's "The Galaxy Is Calling Me!" It will be released along with Delta's Movie 2.
https://www.animenewsnetwork.com/news/2 ... rt/.169225
Don't know when it will take place since the last time we saw Sheryl in "Gorgeous", she was closer to her Delta era age of 25 instead of 17 like in Frontier proper. Neither of them look like teenagers in the image there.
Gorgeous:
https://youtu.be/-KUDHyPn4jk