A sealing gasket for use attached to the topmost edge of gypsum board used as construction wallboard. The gasket includes a support leg section capable of being mounted in engagement with the uppermost edge of the gypsum board. The gasket includes a sealing leg extending outwardly from said leg section which includes an intumescent component to facilitate sealing between the edge of the gypsum board and the wall structure above. The sealing leg preferably includes an intumescent impregnated paper material covered with a structurally enhancing layer. An adhesive layer can be included to attach the gasket to the gypsum wallboard. The sealing leg is preferably planar and extends upwardly and inwardly over the upper edge of the gypsum board at an obtuse angle with respect to the support leg for sealing the head-of-wall area thereabove.