Simply extract the tar ball containing the source into any directory under your web root:
% cd public_html % tar -xvzf gmapsphp-current.tgz gmapsphp-1.2/ gmapsphp-1.2/XMLParser.php gmapsphp-1.2/gmap.inc.php gmapsphp-1.2/sample/ gmapsphp-1.2/sample/visited.xml gmapsphp-1.2/sample/map.cfg gmapsphp-1.2/sample/index.php gmapsphp-1.2/README % mv gmapsphp-1.2 mymap
The distribution comes with a working sample extracted into the sample dirctory.