So.
Musical.ly by itself is a copyright disaster. Musical.ly (lipsyncing to the actual original recordings of songs) is in no way fair use at all. I have no idea how that site continues to exist. (EDIT: after looking through, it seems that the idea is that a musical.ly only uses around ~15 seconds of the original song. There is no safe harbor limit for fair use, but I guess that's better than lip syncing the entire song...that being said, that does raise questions about whether Gabbie's use is fair use.)
Gabbie's reaction to her sister's copyright infringement could potentially be fair use (but it doesn't change the fact that the musical.lys themselves are likely not a fair use), because she only uses bits and pieces (at least, I don't know -- are musical.ly's lip syncs of the entire song or just bits and pieces of a lip sync? [EDIT: see above edit. If the entire musical.ly is only 15 seconds, then Gabbie may be using "too much" of them for it to be a fair use.]), and she only uses those pieces for her criticism (or rather, her imitation).
However, since Gabbie is a pretty big YouTuber who is part of the Collab Creators MCN, it's probably that Collab handles any licensing requirements that she might have. So you can't really judge what she's doing for what you can do, because chances are, you are not part of a network that handles your licensing.