Gestalt is a fancy way of saying "the sum of the whole is greater than its parts." An arm and a leg are nice to have but they generally work better when combined with hands, feet, eyes, ears, and all our other body parts.
What does this have to do with desktop publishing, you ask? In simple terms, a great graphic in an ad may look good but it won't guarantee sales. Combine it with a good caption, great copy, an easy to read headline font, and attention-grabbing color and you're on your way to making more sales than you can handle. It's all those bits and pieces of the ad put together that give it "good gestalt." In desktop publishing you're creating gestalt when you combine elements and principles of design in your page layout.