Except that there is a slight implication in "garish" that the effect is more "in your face" than "gaudy". There is a range of decorated china called Gaudy, http://tinyurl.com/2y3lqn - it's gaudy but not garish.
What is the difference between garish and gaudy? :)
This is belated, and although I agree with Anglika, I would also likely use "garish" in a seedier or underworld context (Tarantino's scenes are garish), whereas "gaudy" I would use only to mean "tacky" or "cheap-looking" in the over-the-top sense!
Just my two bits