The molecular speeds of gaseous molecules are analogous to those of rifle bullets; why is then odour of the gaseous molecule not detected so fast?

No, doubt molecules in the gaseous state move almost with the speed of rifle bullets. But these molecules donot follow a straight path. Since they undergo collisions with one another at a very fast rate, the path followed is zig-zag. Thus, the odour cannot be detected at the same speed at which the molecules move.