I do have a way to make rhythm games in GMS2, it's hard to explain with all code, if you want I can send some example code if you'd like but if so just make sure it's not just stealing, maybe just examine the code and make it in your own way or just credit me.
The basic idea is to use linear interpolation with the place the song is at and where the place you want to either effect something at the beginning or end of that thing's lifetime (represented through variables for these three main values)