Historically speaking, the most important comics serialized in the wartime French comics weekly Gavroche was 'Fantômas', the first-ever comics featuring the phenomenally popular pulp fiction anti-hero of the same name which was created by French writers Marcel Allain (1885-1970) and Pierre Souvestre (1874-1914) in 1911. Between 1911-13, the duo had penned a staggering amount of 32 Fantômas novels, several of which were adapted to the screen in a series of equally succesful movies between 1913-14. In 1926, Allain would revive the character in a news series of books (Souvestre had died in 1914). Hence, the 'Fantômas' comics which kicked off in the central color pages of no. 24 (dated April 10th, 1941) of Gavroche carries a "Marcel Allain presénte" tag.
Below is the scan of the bottom two rows of the first page where Fantômas makes his first appearance (apart from the extra-panel centerfold figure seen in the above scan) in the comics as he dramatically bursts from behind a wall clock (*):
The first installement of 'Fantomas' covered both of the two central color pages:
'Fantômas' lasted till no. 30 (dated May 22d, 1941) of Gavroche and the rest of the installements beyond the first one were apparently single-page per issue, so there are a total of 8 pages. The original art work for one of the pages (**) can be seen here:
The identity of Mat, Tori's companion in 'Fantômas', is less clear-cut. It is tempting to identify him as the leading French comics artist Marcel Turlin (1895-1982) who used Mat as his penname. However, an online French comics dbase, la database BD du Loup, notes that Spanish emigre artist Martí Bas (1910-66) has also used the penname Mat in Gavroche. Bas, an illustrator who had produced propaganda posters for the Communist-led Republican government during the Spanish Civil War, was indeed a companion of Clavé even back in Spain where they had collaborated in stage set designs in Barcelona.