Gah is one of Timmy's alter-egos. When he wished for the body of a 16 year old, but still has the mind of his 10 year old body, Timmy became "Gah!", a Norwegian supermodel.
Gah made his appearance when Timmy wished for a body of a 16 year old, but would still have the mind of his 10 year old body, so that he could ride on the Heart Stopper at Adrenaland. Vicky found him, began to crush on him, and when she asked him what was his name was, Timmy yelled "Gah!" in surprise. Vicky later says "What kind of name is 'Gah'?" Timmy tells her it's a Norwegian name. Throughout the remainder of the episode, "Gah!" is treated as an actual existing celebrity supermodel, even by Timmy's friends and family. A few people are seen taking pictures of him and selling on the Internet. People also buy Gah calendars, dolls and other merchandise off of it too.
Gah looks mostly like Timmy, but with longer hair, stubby facial hair, a buff body, and the looks of a Norwegian supermodel.
In the episode, "Love at First Height", Timmy and his friends visit the Adrenaland amusement park, but become disappointed when they are deemed too small to go on certain rides. While Chester and A.J. split from Timmy to sneak into Adult Land, Timmy uses his fairies to wish himself the body of a sixteen year old, but keeping his normal ten year old mind. While trying to ride the Heartstopper roller coaster, Vicky hops in the cart with him, prompting him to yell "Gah!" in response to her asking him his name, and from hence forth he is known as "Gah!". Vicky becomes attracted to the poor fellow and continues to drag Gah throughout the park while chasing off the growing amount of admirers toward him, who treat him as if he is really a Norwegian supermodel like Timmy said.
Eventually, Timmy manages to wish himself back to normal, and Vicky goes all the way to Norway (or rather, the North Pole) looking for her Norwegian supermodel (since Timmy claimed to her Gah went back to Norway, which he mispronounced as "Norwegia"). Gah does not appear again, and Timmy's normal sixteen year old form seen in Channel Chasers is quite different and much less buff than Gah.