I think the best easy way in this case is to use parseToStringDate which is part of GDK (Groovy JDK enhancements):
Parse a String matching the pattern EEE MMM dd HH:mm:ss zzz yyyy containing US-locale-constants only (e.g. Sat for Saturdays). Such a string is generated by the toString method of Date
Example:
println(Date.parseToStringDate("Tue Aug 10 16:02:43 PST 2010").format('MM-dd-yyyy'))