@gintamacountry I think you're right and wrong, it's true nowadays that you don't see musicians throw around the term "concert" very often, but it's not that they don't wanna use it lightly, it's more that the term went out of fashion, so when it's used today the meaning is a bit different. Like Musician-A says they have a concert to distinguish it from the usual bar gig and make it sound more gransiose, that's fine but in the end, the words mean the same thing. If you know any jazz musicians (or look up some recordings of classic jazz performances) they might tell you that back in those days the term concert was more common than show or gig, so it's probably the better word in the context of this manga.
Hope that helps.
Edit: If you really wanna dig into it the term gig probably came from jazz too so it was more of a transitional period between classic and jazz. so both could be fine anyway. Look into it if it really bothers you