To display kml, simply pass the url containing your data with the required options if needed. Options are listed in the google maps documentation.
<%= gmaps( :kml => {:data => '[{ url: "http://www.searcharoo.net/SearchKml/newyork.kml"}, { url: "http://gmaps-samples.googlecode.com/svn/trunk/ggeoxml/cta.kml", options: {clickable: false } }]' } ) %>
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