gbx to dxf conversion is most likely related to export or conversion of gerber/CAD files, considering that this is the only type of .gbx files that makes sense for any kind DXF export. If you do not have access to the original tool that has been used to save these files in first place, your only alternative is to rely on some dedicated gerber converters, for example the Gerbview.
Updated: July 19, 2023