He is a renowned and extremely talented chef who wrote the bestseller Anyone Can Cook and founded the restaurant Gusteau's, which merited five stars. His personal motto is that anyone can cook, and is prone to producing recipes with unusual flairs. He was the youngest chef in history to obtain a 5-star restaurant, and was among the finest chefs worldwide.
After France's top food critic Anton Ego gave his restaurant an insulting review which led to the loss of one star rating from the restaurant (for reasons unknown, as Ego was just one man compared to the masses who greatly admired Gusteau's cooking), the review broke Gusteau's heart and he died shortly afterwards, which, according to tradition, resulted in the loss of another rating, meriting the restaurant three stars. Gusteau's ownership of the restaurant was handed over to his sous chef Skinner, who was not exactly faithful to his boss's ideals. He later appears as Remy's guide, leading him to Gusteau's.
It is later revealed that Alfredo Linguini is really the son of Gusteau. He is the son of Renata Linguini, with whom Gusteau once shared a romantic relationship. Linguini later acquires the restaurant after Remy informs him of this discovery.
Unfortunately, Skinner tries to sabotage Linguini's attempts to please returning customer Anton Ego by kidnapping Remy. Remy talks to Gusteau in his cage and is frustrated because he feels that all this time he has been pretending to be things he is not, like a rat for his dad and a human for Linguini, and that he (Gusteau) is really there for him to talk to. In his final appearance to Remy, Gusteau tells Remy that he doesn't need to pretend, and that he never did. He was being his true self the whole time. Uplifted by these words, Remy is freed and rushes to Linguini's aid, and the two succeed in impressing Ego and start a new partnership in which they remain respected chefs.
Gusteau does not appear again after Remy's kidnapping. Despite the eventual closing of his restaurant, however, his legacy lives on in Linguini and Remy, as they continue to cook astounding meals in a new restaurant appropriately named La Ratatouille.
His name and surname are anagrams of each other: Auguste (gusteAu) and Gusteau (auGuste).
In the original screenplay, Gusteau was still alive, but Brad Bird concluded there were too many stories to tell and decided Gusteau would be dead. However, he does still speak to Remy as his "conscience." When Remy asks how a man can communicate with a rat, Auguste admits that he is a figment of Remy's imagination and it is Remy's mind that is channeling him from the great beyond.