Fantasio (of Spirou et Fantasio) is one of the supporting characters of the series. Being Gaston's superior, we generally see him when he is trying to sign contracts with Monsieur De Mesmaeker, without ever succeeding because of Gaston’s interruptions… What is astonishing about this character is that in his adventures with Spirou, he symbolizes fantasy, and invention, contrasting with the serious nature of Spirou. But in the Gaston adventures, he plays straight man to Gaston's goofy antics, who thus becomes to Fantasio what Fantasio is to Spirou!

Spirou (also from Spirou et Fantasio) appeares time to time, usually in conjunction with Fantasio. Both Fantasio and Spirou left that series when André Franquin left Spirou et Fantasio after the album Panade à Champignac.

Léon Prunelle is Fantasio’s successor in the post of Gaston’s boss, and continues the futile attempts to sign contracts with De Mesmaeker. A short-tempered man, he has declared he would be "The Man Who Made Gaston Work", and in this capacity continually hunts Gaston down to prevent him from napping or taking a break, and to remind him that he must tend to the backlog of mail. He also tries to keep him from cooking, from tinkering, from inventing, and, obviously, from "gaffing". When he loses his temper his face often turns reddish purple and he utters his trademark outburst "Rogntudjuuu !" (a mangled version of "Nom de Dieu", then not acceptable in a comic also read by children).

Yves Lebrac is one of the cartoonists. He is much more laid-back than Prunelle and has a tendency to lose his work implements, such as his eraser, his pencil, his India ink, etc. — Gaston or his cat often have something to do with this. He continually tries to make passes at one of the secretaries, but without success. He is also fond of comical word games, which all too often annoy Prunelle.

Monsieur Boulier, a surly accountant for the Editions Dupuis publishers, will not rest until he tracks down every useless expense for the publication and more particularly Gaston’s.

Mademoiselle Jeanne ("M'oiselle Jeanne" for Gaston) is one of Gaston’s colleagues, but she occupies a special place. In fact, she is in love with Gaston, who is equally in love with her.

Jeanne has an undying admiration for Gaston. She admires his talent, his courage, his capacity to dare to try the craziest of things, etc. As for Gaston, he has gone so far as to borrow the fire department’s big ladder (courtesy of one his friends) in order to pay her a visit when she was having a tiff with her mother. He does succeed in disappointing her from time to time. Once, she believed that he was carving a heart with their initials in the bark of a tree, but Gaston was in fact in the process of drawing a face.

She is often portrayed in a "damsel-in-distress" role in Gaston's daydreaming episodes, where he unfailingly appears as heroic until confronted with some symbolic foe (usually unprocessed mail!).

However their relationship is mostly platonic. They address each other with the formal vous, see each other mainly at the office, and only rarely go out together.

At first, she was very shy, and drawn in a very classic style, but she progressively evolved into a character much more like the rest of the group.