The audio that you link in Magma will start playing immediately when the song starts — it doesn’t know that you left a gap in Reaper. You can drag the left edge of your audio file in Reaper and export it to a wav file, and then use that in Magma.
Might want to put some cymbal hits in there while you’re at it for lead-in.
You are about to leave Rhythm Gaming World and access the external songs DB. While no song or file sold for commercially available videogames is hosted there and only fan generated content is allowed, Rhythm Gaming World has no direct control over the database and no files are hosted here. Alright, we had to made you aware of this, now you can visit the database!