groovy-vfs makes downloading & unpacking very simple. The following works well for smaller archives. Once the archive contains thousands of files, it is better to download first and then unpack. (This is due to a performance issue inside VFS2, not groovy-vfs).

Download+Unpack.groovy

@Grapes([
  @GrabResolver( name='grysb33r', root='http://dl.bintray.com/ysb33r/grysb33r' ),
  @Grab( 'org.ysb33r.groovy:groovy-vfs:0.2' ),
  @Grab( 'commons-httpclient:commons-httpclient:3.1')
])

import org.ysb33r.groovy.dsl.vfs.VFS

// This will download the compressed archive from Sourceforge and unpack it to a local directory
new VFS() {
  cp  "tgz:http://downloads.sourceforge.net/project/nant/nant/0.92/nant-0.92-bin.tar.gz", 
      new File("./Downloads"), 
      overwrite:true
}