The Grease Pencil is locked to sRGB clamped values AKA default transform, but it’s only in the viewport, your final render will look wrong from what you set up if you were in a different transform (like filmic).
So no way of set up correctly a Grease Pencil in an environement with real light behavior.
I don’t understand why we still introduce inconsistency in the way Blender color management is working.
I would have understand if the render will output the environement in Filmic and Grease Pencil in default, or it would be a choice to limit GP to default transform in viewport AND in render.
Even if in my opinion it’s a bad choice, since GP can have effect like glow… but having different color spaces in viewport and in render it’s really a bad choice. If it’s a choice.