I'm going through the animation courses on cgcookie for Blender 2.79 but I'm struggling finding anything for 2.8 anywhere on the internet. Maybe I'm just impatient.
It is a VERY new feature, ha!
Most videos I've seen about Grease Pencil are devoted mostly to showing off features. I don't think anybody has made any tutorial-like videos about it yet. Blender Cloud will almost certainly have some Grease Pencil courses at some point, but that might not happen until 2.8 is "officially" released (and Blender Cloud needs a subscription, although the money goes to the Blender devs themselves). Aside from that I'm sure there'll be some YouTube tutorials at some point, but I have no idea when. As hard as it might be, you might just have to wait! :)
Or explore it and learn by trial and error...
I do not know if CGC is planning any courses for the Grease Pencil feature.....Grease Pencil rides this weird gray area because it's a Blender feature, but it's definitely in the realm of 2D Concept rather than 3D.