The method for producing a gas separation composite membrane includes applying a mixed liquid containing compounds (a) and (b) below onto a porous support to form a coating film and curing the coating film to form a crosslinked polysiloxane compound layer: (a) a particular crosslinkable polysiloxane compound having a structural unit (a1), a structural unit (a2), and a structural unit (a3) or (a4), and (b) a particular crosslinkable polysiloxane compound having a structural unit (b1), a structural unit (b2), and a structural unit (b3) or (b4), where R1a to R1f and R2a to R2f represent a particular group and * represents a particular linking site.