getRawOffset() returns the amount of milliseconds relative to UTC (which
I believe is (almost) the same as GMT). Some dividing by 1000 and 60 and
you could end up with the format you want.
<http://java.sun.com/j2se/1.5.0/docs/api/java/util/TimeZone.html#getRawOffset()>

    TimeZone tz = ...
    off = tz.getOffset(new Date().getTime()); // considering DST

or
    off = tz.rawOffset(); // ignoring DST

And some simple math to get the hour and minutes from the return value -
which is in milliseconds.
   

/Thomas

You can get close to what you want with SimpleDateFormat (no colon,
though), with the benefit that it adjusts transparently for Daylight
Savings Time rules, e.g.:

jc@rs-dev2:/tmp$ cat tztest.java
public class tztest {
 public static void main(String [] asArgs) {
   java.text.DateFormat df = new java.text.SimpleDateFormat("'GMT 'Z")
;
   for(int i=0; i<asArgs.length; ++i) {
     java.util.TimeZone tz = java.util.TimeZone.getTimeZone(asArgs[i])
;
     df.setTimeZone(tz) ;
     System.out.println(asArgs[i] + " / " + tz.getDisplayName() + ": "
+ df.format(new java.util.Date())) ;
   }
 }
}
jcx@rs-dev2:/tmp$ java -classpath "." tztest America/New_York UTC
America/Los_Angeles
America/New_York / Eastern Standard Time: GMT -0400
UTC / Coordinated Universal Time: GMT +0000
America/Los_Angeles / Pacific Standard Time: GMT -0700