for 2d games, you can export grease pencil animations as pngs and static objects as svgs. Not ideal.

For 3D, you can convert to mesh and bake everything in. Also not ideal.

if we saw game engine impementation for Grease Pencil, I'd hope it was 3D, as the power of Grease Pencil is mostly from what you can do with that Z axis. If an engine like Godot implemented it, that would be ideal. As Godot's 2D is actually 3D iirc.