Rendering a 'google_map_field' when referenced via entity reference field
I would recommend using a view mode to render out the referenced entity. 

For instance, A's display settings is set to display B using the 'teaser' view mode. Then, you can configure B's teaser display to show what you want.

I know this doesn't answer your direct referenced entity value question, but this is how I would tackle this problem.

I hope that helps!