build/gant.xml

<project name="Test gant invocation from Ant script" default="doGant">
  <dirname property="build.dir" file="${ant.file}"/>
  <property name="project.home" value="${build.dir}/.."/>
  <property name="gant.home" value="${project.home}/build/lib/gant"/>
  <property name="gant.target" value="default"/>

  <path id="gant.classpath">
    <fileset dir="${gant.home}/lib">
      <include name="*.jar"/>
    </fileset>
  </path>

  <taskdef name="gant" classname="org.codehaus.gant.ant.Gant">
    <classpath refid="gant.classpath"/>
  </taskdef>

  <target name="doGant">
    <gant file="${gant.script}" target="${gant.target}"/>
  </target>
</project>