How do I display JSON data and Location module data on the same Gmap?

How do I display JSON data with latitude/longitude values on the same Gmap view showing data from the Location module? I know how to parse JSON into an array, but I don't know how to use this array in the module to achieve what I want.

My objective is to show external content from an API in the same Gmap view. This could also be used to show content from any sites like Foursquare via an API as long as it has latitude/longitude values. For example, I could make a map that shows "Foursquare content nearby" for the currently-viewed node, but also use this same map to show nearby Drupal nodes.