Below have:
1 - Log of Gradle failing
2 - Log of Groovy passing
3 - Gradle code
4 - Groovy code

1 - Log of Gradle failing =====================================================================
carlton@laptopDellLatD820:~/cjMounts/local/boot/sdx5_mainStore/cj2010work/code/gradleBug1$ gradle
:runs
return code: 0
stderr:
stdout: bob
return code: 0
stderr:
stdout: bob

BUILD SUCCESSFUL

Total time: 3.343 secs

2 - Log of Groovy passing =====================================================================
carlton@laptopDellLatD820:~/cjMounts/local/boot/sdx5_mainStore/cj2010work/code/gradleBug1$ groovy build.groovy
return code: 0
stderr:
stdout: bob
return code: 0
stderr:
stdout: fred

3 - Gradle code ===============================================================================
defaultTasks "runs"

task runs << {
execAnt("echo","bob")
execAnt("echo","fred")
}

String execAnt(String cmd, String arguments) {
ant.exec(outputproperty: "cmdOut",
errorproperty: "cmdErr",
resultproperty:"cmdExit",
failonerror: "true",
executable: cmd)

println "return code: ${ant.project.properties.cmdExit}"
println "stderr: ${ant.project.properties.cmdErr}"
println "stdout: ${ant.project.properties.cmdOut}"
}

4 - Groovy code ===============================================================================
String runs () {
execAnt("echo","bob")
execAnt("echo","fred")
}

String execAnt(String cmd, String arguments) {
ant = new AntBuilder()
ant.exec(outputproperty: "cmdOut",
errorproperty: "cmdErr",
resultproperty:"cmdExit",
failonerror: "true",
executable: cmd)

println "return code: ${ant.project.properties.cmdExit}"
println "stderr: ${ant.project.properties.cmdErr}"
println "stdout: ${ant.project.properties.cmdOut}"
}

runs()