Add linked geometry node asset via script

A function that add a GN modifier to a mesh object and assign a specific node from a blend file.

import bpy
from os.path import join as os_path_join

def add_gn_modifier_with(obj, file_path, node_name, method="LINK"):
    if hasattr(obj, "type"):
        if obj.type != "MESH":
            print("MESH object only")
            return
    else:
        print("Invalid object")
        return

    if file_path == bpy.data.filepath:
        print("Not allow load library from current file")
        # Don't append current file, It will crash blender even the code inside the try:
        return

    inner_path = "NodeTree"
    node_groups = bpy.data.node_groups

    def wm_append(link=True):
        try:
            bpy.ops.wm.append(
                filepath=os_path_join(file_path, inner_path, node_name),
                directory=os_path_join(file_path, inner_path),
                filename=node_name,
                link=link)
        except Exception as ex:
            # file_path or node_name may wrong
            print(str(ex))
            return False
        return True

    if method == "LINK":
        name_tuple = node_name, file_path

        if name_tuple in node_groups:
            pass
            # Already Linked
        else:
            if not wm_append(link=True):
                return

        if name_tuple in node_groups:
            node = node_groups[name_tuple]
        else:
            print("node group not found in the file_path")
            return

    elif method == "APPEND":
        if node_name in node_groups:
            old_node_groups = set(node_groups)

            if not wm_append(link=False):
                return

            try:
                node = next(ob  for ob in node_groups  if ob not in old_node_groups)
            except:
                print("node group not found in the file_path")
                return
        else:
            if not wm_append(link=False):
                return

            if node_name in node_groups:
                node = node_groups[node_name]
            else:
                print("node group not found in the file_path")
                return

    elif method == "REUSE":
        print("Unsupport method")
        return
    else:
        print("Unsupport method")
        return

    if node.type != "GEOMETRY":
        node_groups.remove(node)
        print("Wrong Node Type")
        # you may want to check if the node marked as asset
        return

    md = obj.modifiers.new(node_name, "NODES")
    md.node_group = node