That's very easy:
var map = new GMap2(document.getElementById('map_canvas'));
var directions = new GDirections(map);
directions.load('from: London, UK to: Glasgow, UK');
Screenshot:
UPDATE:
Using the v3 API is a bit more verbose, but still straightforward:
var map = new google.maps.Map(document.getElementById('map_canvas'), {
mapTypeId: google.maps.MapTypeId.ROADMAP
});
var directionsService = new google.maps.DirectionsService();
var directionsDisplay = new google.maps.DirectionsRenderer();
directionsDisplay.setMap(map);
var request = {
origin: 'London, UK',
destination: 'Glasgow, UK',
travelMode: google.maps.DirectionsTravelMode.DRIVING
};
directionsService.route(request, function(response, status) {
if (status == google.maps.DirectionsStatus.OK) {
directionsDisplay.setDirections(response);
}
});