users_controller.rb

## users_controller.rb
class UsersController < ApplicationController

  respond_to :html, :json

  def index
    @json = User.all.to_gmaps4rails
    respond_with @json
  end

end

## view.html.erb
<!-- create html + load js files but don't create map itself: will be done after ajax call -->
<%= gmaps({:last_map => false}) %>

<!--  button to trigger ajax call -->
<button type="button" id="ajax">Load Map</button>

<script type="text/javascript" charset="utf-8">
$(function() {

//hide the empty container
$(".map_container").hide();

$("#ajax").click(function(){

  $.getJSON('/users', function(json){
    $(".map_container").show();
    Gmaps.loadMaps();
    Gmaps.map.addMarkers(json);
  })
  })

});
</script>