The invention relates to a method and a device (16) for gas filling and sealing of a duct (7) intended to be filled with gas and positioned in a container (1) of a collapsible type. The duct (7) is defined by two opposite side walls (2) which are joined along a connecting portion (4) and comprises an inlet (15) arranged in one of the side walls (2). A part of the container (1) that comprises the inlet (15) is clamped between an abutment (17) and a gas module (20) which is axially movable towards the abutment (17). Clamping occurs in such a manner that one of the two side walls (2) included in the duct (7) is allowed, in response to a gas flow supplied from the gas module (20) and entering the duct (7) through said inlet (15), to bulge to form a free passage into the duct (7) for filling the same with gas. After completion of the gas filling, the duct (7) is sealed. The invention also relates to a container blank (10) having a duct (7) which is specially developed for the method and the device (16).