This is just a quicky that might be useful to others, too. The following function unzips the input to the output.
Update: As Ben pointed out, tis will only work correctly for gzipped text files encoded in UTF-8 as input (ASCII, ISO-5589-1 will also be fine).
(ns foobar
(:require [clojure.java.io :as io]))
(defn gunzip
[fi fo]
(with-open [i (io/reader
(java.util.zip.GZIPInputStream.
(io/input-stream fi)))
o (java.io.PrintWriter. (io/writer fo))]
(doseq [l (line-seq i)]
(.println o l))))