I love CEL shading. Toon stuff. But. Like. If I really really wanted a gradient I'd dump all the images in CSP or Krita and hand draw. I mean. That's kind of what happens here at 1:29. But. At least when I'm done with a frame I can render 001 w a gradient and go on with life all inside Blender. I feel, like Grease Pencil is working like a locked version of sculpt. I just noticed this. So. I think that's why it does certain things a certain why counter intuitive to any other paint program. So. You go on a bit of a walk for something that's like two clicks in another program.
Well it's different, because it's 2D inside of a 3D environment. Also a little history helps to understand why GP is they way it is. It started as a digital version of the old grease pencils that were used to draw on the glass of old style monitors in the early days of computer animation. Thus the name Grease Pencil. Then people started using it for storyboarding and quick animations. So, they decided to make it into a 2D animation system and the old GP in now called the Annotation Tool. It's design also has to be flexible enough to draw using 3D meshes as the canvas and be interactive with the 3D environment.
Most of the time when I've used the gradient in the grease pencil, it's been more for backgrounds than for parts of the actual sketch because it's hard to get the gradients to do just right in Grease Pencil if you're tryng to use them for shading I think.
@Dillenbata3 That's a neat quick history. I didn't know that. It just seemed like a compact or restricted version of another mode. It feels very Sculpt Mode or Edit Mode but with vertex paint as like a modifier. gradyp I did see Blender Studios Cow Anime. They solved it for a short. It looked similar to what I've done. When the shadow drifts do a hold or add an inbetween. Cageggi's comic book material does really nice halftones but it uses a ton of VRAM for all the nodes. TL;DR it looks gradients and halftones are easier now but still kind of an art form.
Oh, I know GP can create some gradients... But gradients work mostly in either a completely spherical or in a straight line, so getting them to curve in an organic way takes a lot of work.