This snippet in your pom should give you a better idea of what's available while running the script. Most of the interesting bits are probably in the binding.project, an instance of MavenProject.
<build>
<plugins>
<plugin>
<groupId>org.codehaus.groovy.maven</groupId>
<artifactId>gmaven-plugin</artifactId>
<executions>
<execution>
<phase>generate-resources</phase>
<goals>
<goal>execute</goal>
</goals>
<configuration>
<properties>
<hello>world</hello>
</properties>
<source>
println this.binding.variables
println project.properties
println settings.properties
</source>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>