Google_authentication is a project mainly written in Java, it's free.
how to implement authentication against google LDAP
To run this project you have to :
mvn clean compile mvn exec:java -Dexec.mainClass=com.majecek.gdata.App -Dexec.args="username[without @gmail.com] password"
some tips how to run maven to execute java code (requires to modify pom.xml) http://www.vineetmanohar.com/2009/11/3-ways-to-run-java-main-from-maven/