It all started at a crab-apple fair in rural England way back in 1269. King Henry III himself had given the fair a royal charter, so I suppose a lot of people felt compelled to eat the crab-apples, which, being ridiculously bitter, made the eaters pull funny faces, and for a bit of fun, they decided to make a contest out of it. Almost 800 years later, the crab-apple fair is still an annual event, as is the gurning contest, although nowadays it draws contestants from all over the world. (The etymology of "gurn" is a bit muddy, though the venerable OED guesses that its provenance might be Scottish, and related to the word "grin." In Northern Ireland, on the other hand, the word has a very different meaning -- "to cry.")
Over the years, serious gurners have developed a number of winning strategies, the most effective of which is to have no teeth, which makes one's facial features much easier to warp. England's best-known gurner, Peter Jackman, had his teeth removed in 2000 to facilitate extreme gurning (even though he had already won the world championship four times -- so dogged was he in the pursuit of gurn-fection). Three years later, he died in a golfing accident. Which is nothing to gurn about.
I found two great videos of gurning competitions, the first from the 1960s, the second from the 80s. Something that won't make sense unless I explain it: it's tradition for gurners to gurn with their faces framed by a horse collar, known as "gurnin' through a braffin'."
Image by thebrier.