grp to mp3 conversion is typically searched by users that own some old guitar music notation in .gtp format (Guitar Pro) and would like to turn that into music in MP3 format, for example for making ringtones.
This is actually doable with any music notation software that supports Guitar Pro formats, as export to MP3 is common feature. Just keep in mind that the result is essentially MIDI audio turned to MP3.
Updated: August 28, 2023