code: def gdf_to_geojson(gdf, date_format='epoch', properties=None, filename=None):
      
  
      # convert dates/datetimes to preferred string format if specified
      gdf = convert_date_columns(gdf, date_format)
  
      gdf_out = gdf[['geometry'] + properties or []]
  
      geojson_str = gdf_out.to_json()
  
      if filename:
          with codecs.open(filename, "w", "utf-8-sig") as f:
              f.write(geojson_str)
          return None
      else:
          return json.loads(geojson_str)
docs: Serialize a GeoPandas dataframe to a geojson format Python dictionary / file