The only Gaddis book I've read is A Frolic of His Own. It is essentially nothing but dialogue among upper class lawyers (& associated ilk), and I won't pretend it's a page-turner, but my word, it was masterfully crafted. Dialogue in novels is often so artificial, yet he managed to make it not only accurate and natural but also perfectly adept at carrying the novel along as well as if it were written in standard prose.