Someone wrote


{
      "v" : integer, // file struct version
      't' : integer, // geometry type id (point/line/poly)
      "g" : [ // array of geometries
          {
              "c" : integer, // points count
              "m" : [ // array of metadata
                  {
                      "t" : integer, // data type id (integer/double)
                      "n" : string, // gpx tag name
                      "d" : string // points meta (base64)
                  }
                  ...
              ]
          }
          ...
      ]
  }