Coming from a Nintendo Life interview with dev Tak Fujii...
NL: When was it decided to make Gal Metal exclusive on the Switch?
TF: Gal Metal is still a single player game, at the moment. Since there are no note indicators on screen, players have no need to focus on the screen, but instead listen to the audio from the console. This means other players also have no need to focus on the screen. Watching other players play is the most fun part of this game in my opinion. Remember most fun part of playing Just Dance? It's us, observing players. With the Joy-Con and screen-less game play experience, Gal Metal has no other home but Switch.
NL: Regarding the actual music, is the whole game an original soundtrack?
TF: They are all covers from classical music. It's great to have actual recordings in the game like other music games. However, my focus in Gal Metal is 'Build your rhythms as you like". Therefore we haven't use any existing sound tracks, which allows players to think and build their favorite rhythms. Again, building and improving music is one of the big core ideas of my design and I really want players to feel this. Additionally, people don't like to play unknown songs so I picked famous classical pieces from the past 400 years, which are no doubt recognised as 'great pieces of music'. Because they are famous, the player should be aware of how and when they build up in the song. The player may have instant ideas about what to do immediately.