One possible option could be Penny Fitzgerald. Penny is a recurring character in The Amazing World of Gumball and is primarily depicted as a love interest for Gumball. However, since you are suggesting her as SpongeBob's girlfriend, this would add an interesting twist to the story.

Another option could be Masami Yoshida, a cloud-like character in The Amazing World of Gumball. Masami is well-known for being popular and often gets attention from the other characters. However, in your scenario, she could be SpongeBob's girlfriend, bringing a new dynamic to the show.