Gum in an early appearance
Gum is a running gag in Calvin and Hobbes. Calvin enjoys reading Chewing Magazine, and elevates gum chewing to the status of a sport. Gum was mostly featured in later strips that followed 5/4 through 5/8 1992's introduction of Chewing.
Magazines
Main article: Chewing Magazine (Chewing is categorized as a separate running gag)
Gum chewing magazines were introduced alongside Chewing in the 5/4 through 5/8 1992 story arc. When Hobbes expressed disbelief for the existence of a gum chewing magazine, Calvin replied that had to be a dozen such publications, each marketed to a different class of gum chewers.
Calvin named two of them: Gum Action, which he said "aimed for the gonzo chewers", and Chewers Illustrated, which he claimed was targeted at collectors of vintage gum.