path stroke

HTML

<script src="//www.google.com/jsapi"></script>
<div id="visualization"></div>

CSS

#visualization g[clip-path] > g + g > g > path {
    stroke-width: 0;
    fill: transparent;
}

JavaScript

google.load('visualization', '1.1', {packages: ['geochart'], callback: drawVisualization});

function drawVisualization() {
    var data = google.visualization.arrayToDataTable([
        ['Country', 'Value', {role: 'tooltip', p:{html:true}}],
        ['US', 5, 'Hello world'],
       ]);
    var chart = new google.visualization.GeoChart(document.getElementById('visualization'));
    chart.draw(data, {
        width: 800,
        height: 600,
        region:'world',
        tooltip: {
            isHtml: true
        }
    });
}