You should be able to use the output attribute to direct the output of a groovy ant task to a file:

 
<groovy output="logFile.txt">
.....
</groovy>
 

I could not get this working with groovy version 1.5.6 though. So I use the following statements within my groovy code to direct the standard output to a file:

 
def stdOut = System.out; // save old System.out
def newFileOutputStream = new FileOutputStream("myLogFile.txt", true);
def newOutputFilePrintSteam = new PrintStream(newFileOutputStream); 
System.setOut(newOutputFilePrintSteam);
...............
System.setOut(stdOut); // go back to default standard out
 

The first and the last statement prevent other groovy tasks in your ant build file logging to the same file.