$headContent
You need to add this as server in your $home/.m2/settings.xml
Here is example:
<settings>
<servers>
....
<server>
<id>appengine.google.com</id>
<username>MyEmail@gmail.com</username>
<password>MyPassword</password>
</server>
</servers>
<profiles>
....
</profiles>
</settings>
Add the reference to server id.
<project>
[...]
<build>
<plugins>
[...]
<plugin>
<groupId>net.kindleit</groupId>
<artifactId>maven-gae-plugin</artifactId>
<version>[plugin version]</version>
<configuration>
<serverId>appengine.google.com</serverId>
<sdkDir>${appengine.sdk.root}</sdkDir>
<appDir>${basedir}/war</appDir>
</configuration>
</plugin>
</plugins>
</build>
[...]
</project>
It is recommented to have passwords in settings.xml encrypted. See how to do this Maven Password Encryption guide