As @Steven points out, a better way would be:
public void writeToFile(def directory, def fileName, def extension, def infoList) {
new File("$directory/$fileName$extension").withWriter { out ->
infoList.each {
out.println it
}
}
}
As this handles the line separator for you, and handles closing the writer as well
(and doesn't open and close the file each time you write a line, which could be slow in your original version)