Most humans, at least most humans who read books and like learning knew words, know that to be petrified means to be so scared and terrified of something that you are paralyzed and can't move. It comes from petra, the Latin word for a stone, and so to be petrified literally means turned to stone. But for gomes the word has a whole different sense to it. Gomes like to be petrified!
Gomes like to sneak around causing mischief while they are in their living, moving, animated form, but if they sense danger, or want to avoid being noticed they can instantly turn themselves into stone... they petrify. When Great Uncle Hugh McDougall gives Finn a new present in The Rusty Pelican he calls it a "very special Scottish paperweight." Finn is not very impressed. It is an ugly, smelly, heavy cylindrical thing with one flat end, one pointy end, and a covering of moss in the middle. But when Wullie comes to life Finn finds him to be much more interesting... and much more trouble.
Most children are curious, have a good imagination and sharp eyes, so they have little trouble seeing gomes. But many adults have no imagination and even less sense of fun, so when they appear on the scene most gomes will petrify, and stay as a rock until the boring grown up goes away. Gomes can also petrify to protect themselves from damage when they jump off roofs, are flung through windows, or when they get attacked by cat claws or slobbering dog fangs. Interestingly, when a gome is in the petrified state it can still hear and see and think perfectly well. In fact Wullie says he can think more clearly and solve more puzzles when he is petrified, without the distraction of being "alive." He just doesn't need to bother with conversation, is protected from attack, and is able to conserve his energy better; to rest. It is a bit like when a tortoise is attacked it withdraws inside its shell. A gome simply petrifies.
How cool would that be, if you could petrify when grown ups are annoying you?
"Jennifer, I told you to clean up your room. Jennifer? Now where is that girl? There's nothing here except a large statue standing in amongst the piles of clothes, books, toys, pop cans and dirty plates. Hmmmm?"
Now, I know what you are thinking, surely the grown up would recognize that the statue looked like you, but a lot of grown ups aren't that smart. They might think it is the statue of a fashion model or a rock star that you bought off e-Bay. Actually if your room was as untidy as this one you could even hide a full sized petrified version of your self under the piles of dirty clothes.
Animals, of course, being smarter than your average grown up, recognize the difference between a rock and a gome even when the gome is in its petrified state. So Wullie gets scratched by Ying Yang, the Siamese cat, and gets buried in the garden by Winston and Riley. When he escapes their clutches and comes back to life he is covered in grass, dirt, and smelly dog slobber, which makes him even more scruffy, rude, and bad-tempered than usual...