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