The GPanel tool is used to turn Grease Pencil strokes into Panel Decals.
Check out MACHIN3tools here and easily create grease pencil objects primed for surface drawing from its Modes Pie.
Grease pencil object with and active layer.
Default: Create panel decal from strokes as they are
SHIFT: Conntect strokes in their order of creation, then create panel decal from them
CTRL: Conntect strokes in their order of creation and make them cyclic, then create panel decal