Integrate GEOJSON inside Google Maps WORDPRESS

Hi, we run a directory/listing website with MyListing Theme and want to add a Geojson layer on google maps.
We tried to target the element but we receive error, can you help us?

Code we tried:
//load geojson layer
map.data.loadGeoJson('milano.geojson');

Error we get: Uncaught ReferenceError: map is not defined