The French word applied to a lying or recumbent effigy on funerary monuments from the 15th century and onwards. More often than not the gisant represented a person in death. In some instances, the person was depicted as decomposing. The gisant was then contrasted with the orant, which is representation the person as if alive.
As funerary art, gisant is not to be mistaken completely with vanitas and memento mori, which are more direct reminders for us to live while we have life to live.
Stating the fact that we’re mortal and that some already are dead, is nothing new in art. It is one of the central themes in art right from the beginning of civilisation and right up to the contemporary art of, say, Damien Hirst. You can read more mortality and art along this path.