To use a fluid layout (this assumes you're using Boostrap 3.x), simply change the options classes to incorporate bootstrap's row-fluid class

<script> 
$(document).ready(function(){   
   $("#mycanvas").gridmanager({ 
        controlPrepend: "<div class='row-fluid'><div class='col-md-12'><div id='gm-addnew' class='btn-group '>", 
        rowClass:    "row-fluid",
        rowSelector: "div.row-fluid",
        rowPrepend:  "<div class='row-fluid gm-editing'>"
    }  
  ); 
});
</script>