Convert mesh/curve into grease pencil for 2.8

obj2gp_28.py

import bpy
from mathutils import Vector

def convert_to_GP(context, object = None, use_one_GP = True):
    if object == None:
        objects = context.selected_objects
    else:
        if isinstance(object,list):
            objects = object
        else:
            objects = [object]
    bpy.ops.object.select_all(action='DESELECT')
    grease = None
    for obj in objects:

        if use_one_GP and not grease == None:
            grease = grease
        else:
            bpy.ops.object.gpencil_add(type='EMPTY')
        grease = context.view_layer.objects.active
        if obj.type == 'MESH':
            obj.select_set(True)
            context.view_layer.objects.active = obj
            bpy.ops.object.convert(target='CURVE')
            obj.select_set(False)
            context.view_layer.objects.active = grease
    
        if len(grease.data.layers) == 0 or use_one_GP:
            layer = grease.data.layers.new(name=obj.name)
        else:
            layer = grease.data.layers[0]
        grease.data.layers.active = layer
        frame = layer.frames.new(context.scene.frame_current)
    
        if obj.type == 'MESH': #if conversion to curve failed
            mesh = obj.data
            for edge in mesh:
                stroke = frame.strokes.new()
                stroke.points.add(2)
                stroke.display_mode = '3DSPACE'
                stroke.material_index = 0
                
                stroke.points[0].co = obj.matrix_world @ mesh.vertices[edge.vertices[0]].co
                stroke.points[1].co = obj.matrix_world @ mesh.vertices[edge.vertices[1]].co
             
        else:
            for spline in obj.data.splines:
                stroke = frame.strokes.new() 
                if spline.type == 'BEZIER':
                    points = spline.bezier_points
                else:
                    points = spline.points
                stroke.points.add(len(points))
                stroke.draw_cyclic = spline.use_cyclic_u
                stroke.display_mode = '3DSPACE'
                stroke.material_index = 0
                for i, point in enumerate(points):
                    stroke.points[i].co = obj.matrix_world @ point.co.xyz
        #bpy.data.objects.remove(obj)
            
class SimpleOperator(bpy.types.Operator):
    bl_idname = "object.convert_to_grease_pencil"
    bl_label = "Convert Mesh/Curve to Grease Pencil"

    def execute(self, context):
        convert_to_GP(context)
        return {'FINISHED'}

def register():
    bpy.utils.register_class(SimpleOperator)

def unregister():
    bpy.utils.unregister_class(SimpleOperator)

register()  

This code is more intended to use with other code, that's why it has options to introduce other objects apart from the ones in the context