Whitman is actually one of Western Publishing's imprints. Before Western's arrangement with Dell, it released several comics under the Whitman label. Then in the 70s and 80s, Western labelled their non-returnable comics (as opposed to those sold on the newsstand market and returnable if unsold) as Whitman.
For now, the GCD lists under Whitman only those items which were not also released under the Gold Key label. See Gold Key for those comics released under both Gold Key and Whitman imprints.
Also note that that the dates of most Whitman comics produced after 1981 are more of a Western production convention and often do not coincide with the dates the comics were made available. Some comics of this period have no indication of date whatsoever, and are dated by the information provided by Western to The Comic Reader.