Maybe someone versed in Japanese can give a hint if she's seen as a prospect for the team or if this was just some make-a-wish/media event.
As there are other high school options nearer to home, Gunji's move to Kyushu means that she is probably eyeing Saishunkan as a future employer, ... and Saishunkan ought to be interested, too.
As for Gunji's past achievements; according to the video:
1. Elementary School 4th ~ 6th grade: All Japan Elementary School Champion
2. As of September 2018: All Japan Junior Champion (This, incidentally, includes all Juniors up to High School 2nd grade, i.e., up to 16~17 year-olds. Gunji is currently a 1st-year High School pupil).
Other achievements, not mentioned in the video, include:
1. As a 2nd-year JHS pupil: 3rd place, Asia Junior U-15
2. As a 3rd-year JHS pupil: Runner-up, Asia Junior U-17
3. As a 1st-year HS pupil: 3rd place, All Japan High School Championships (Mizui Hirari won this)
Importantly, the tournament that featured in this video was none other than the All Japan Championships (AJC). Although she lost in R1 to Mine (21-13, 21-10) , Gunji earned her place here by virtue of having become the All Japan Junior Champion, which is a clear indicator of promise. Actually, even Yamaguchi commented that it is difficult to read whether Gunji is going to return either straight- or cross-shots, and this echoes what Gunji's class-mates said at the beginning of the video. Her form is compact, and therefore unreadable. Gunji, incidentally, attributed her first-round loss at the AJC to a lack of physical strength.
BTW, this is very trivial, but this video features not so much a training session with Saishunkan per se, as a training session for all players in the Kyushu area who would be participating in the AJC, i.e., predominantly Saishunkan players.
As
@ant01 mentioned, Gunji sees Yamaguchi as a role model. She specifically likes the fact that Yamaguchi sets out to "enjoy" badminton, and show that this is an "enjoyable" sport. [... IMO, in stark contrast to Okuhara.]
That aside, how on earth did you manage to come across this video in the first place???