Also known as "whoopie pies" in the less enlightened eastern part of the state (hell, they don't even know what gumbans are). Growing up in Johnstown, I always assumed they were just a regular dessert that everyone had eaten (except myself--I'm allergic to eggs), until I got to college and found that none of my fellow students who came from outside of western PA knew what they were. Now, if you just read the above description and are thoroughly disgusted, don't be--all my friends who lived normal childhoods can vouch that they're quite appetising. They may look disgusting--especially since they're usually squashed as they're wrapped in Saran wrap--but they're supposed to be quite good. While chocolate cake gobs are the generally accepted favourite (and what you'll get by default if you just ask for a "gob"), they are occasionally made from other cake mixes today (most commonly pumpkin, but I've also seen a few vanilla gobs. Do not eat these under any circumstances.).
The story behind gobs
The first gobs were baked by the wives of blue-collar professionals (probably of Eastern-European descent). They would put them in the lunch pails of their husbands, who probably worked as miners or in "still" mills, as an after-lunch dessert treat. Even with most of the mines and mills gone the way of the dodo, the gob lives on as a testament to this more romantic time.
They got their name from one of two sources, depending upon whom you ask. Either from the gobs of icing and cake used to make them, or, more likely, from the picture of a sailor (or "gob") printed on the packaging by one of the first companies to mass-produce them for the commercial market.