Finally, an unused stamp is evaluated for the existence or preservation of the gum on the reverse side. The widespread custom of hinging stamps placed into albums has taken a considerable toll on the population of post office fresh, never hinged stamps, and a substantial portion of extant stamps bear some "scars" of this practice. Decades of handling or environmental hazards have also caused an erosion of the presence of "original gum" and some 19th and early 20th century stamps now have only a fraction or none of their original gum intact. The condition of the gum will appear as a modifier, after the final grade.

Note that minor flaws in the gum such as creases, skips or bends are accounted for in the overall condition or soundness of the stamp. The gum condition modifier refers to the presence or absence of the gum itself and whether or not the stamp has ever been regummed or hinged.