Morph and GIMP:
At the top of the Art overlay, there are three buttons: pencil, paint and hand. The pencil does palm rejection. The paint does not. The hand is to have the overlay function like a mouse, which means the pointer will not jump around; it stays where it is left off.
In pencil mode, the Morph will sound an audible "click" when the pressure is enough to activate drawing or the pressure is released to stop drawing. Paint mode does not. Hand mode "click"s when the overlay is pressed enough for a mouse click (one finger - left mouse button, two fingers - right mouse button).
GIMP 2.8 Input Setup
Launch GIMP and select this menu item, Edit/Input Devices, which will pop up a Configure Input Devices window. You will see Sensel Sensel Morph, Sensel Sensel Morph Pen Pen (0) and Sensel Sensel Morph Touchpad. For each of these, change the Mode from "Disabled" to "Screen", then Save.
Now, select a tool where pressure sensitivity is useful, like Pencil, Paintbrush, Eraser, Airbrush, Clone, Heal, Perspective Clone, Blur/Sharpen, Smudge, or Dodge/Burn. You'll notice under Tool Options that there's a Dynamics section which defaults to Pressure Opacity. While this Dynamics mode will work, it's not a very "natural" media mode. To better mimic a pencil, paintbrush or airbrush, select a different Dynamics mode by clicking on the icon to the left of the word "Dynamics" (three dots with blue arrow). Select either Pencil Generic or Pen Generic. (The other Dynamics modes are probably good for different uses, but for me, these are what I'm after.)
Give it a whirl. You should see the pressure vary the opacity and width of the line.