It includes a number of editorial features, like the first volume's Syntopical Guide and Appendices. One thing it did not include was a table of contents for the entire set, as opposed to individual volumes. You'll find it here on the Center's website. [link fixed -ed.] This, and other information about the set, is now also available at Wikipedia.
Appendix I is a Plan of Graded Reading, which is in the Google Books edition of Volume 1. Not included are Appendix II, Recommeneded Novels, and Appendix III, Recommended Anthologies of Poetry, works not contained in the set.
Just in case someone is looking for suggestions for a third edition of GBWW, Wikipdedia notes that GGB "also contained editorial introductions to the selections, which were generally not included in the Great Books" and "The Gateway volumes were single-column with large, readable type."