Using a Blend Mode and creating a Gradient Map aren't mutually exclusive. You can create a Gradient Map layer and then change the Blend Mode of that layer. Generally speaking, I find Gradient Maps helpful if I want to apply a filter look or color palette to an image in one fell swoop. Whereas creating layers with specific Blend Modes is helpful for achieving shading effects (even as they can ALSO be used to apply filter looks or colors to an image, as described in the video).
