For grease pencil, it isnt completely needed, but I usually use it, so

What I do is I make the grease pencil outline to collection, and then change the size of it to my needing. Then in the effects tab, put pixelation up to 12-15, and turn off anti aisliasing. THEN, If you want the outline to go lighter, I would turn on rim, with the blend mode being normal. In constraints, I turn on child of object, and make it the main feature. Then, with the draw option enabled, I turn on surface mode, and make sure to have an offset of 0.02. NOW ITS ATTACHED AND CAN MOVE WITH THE MAIN HEAD! ! ! ! (If you're rigging, instead of constraints, you would simply rigg the outline with the bone you want it to move with).