gp to xml conversion is typically searched in context of export of older Guitar Pro projects (.gp) to MusicXML format (.xml or .mxl), usually in effort to produce a music notation file that can be used in a plethora of music notation or composition software.
As is often the case of MusicXML format, your best option is the free MuseScore. It should support most of formats from Guitar Pro natively and allow you to export the data to produce MusicXML notation.