The map.addCircle() method adds a circle on the map.
<div class="map" id="map_canvas"></div>
var GOOGLE = {"lat" : 37.422858, "lng" : -122.085065};
var mapDiv = document.getElementById("map_canvas");
var map = plugin.google.maps.Map.getMap(mapDiv);
map.addEventListener(plugin.google.maps.event.MAP_READY, function() {
// Add circle
map.addCircle({
'center': GOOGLE,
'radius': 300,
'strokeColor' : '#AA00FF',
'strokeWidth': 5,
'fillColor' : '#880000'
}, function(circle) {
map.moveCamera({
target: circle.getBounds()
});
});
});