Custom entities in GDScript

Any script can be an entity. Declare a signal for each output and a method for each input, and emit the signal when the thing happens.

@tool
class_name Alarm extends Node3D

signal tripped(activator: Node)

@export var armed := true

func _func_godot_apply_properties(props: Dictionary) -> void:
    armed = GodotTrenchIO.to_bool(props.get("armed", armed))

func trip(activator: Node = null) -> void:
    if armed:
        tripped.emit(activator)

func disarm() -> void:
    armed = false

Make the activator the first Node argument of your signals, so chains further down still know who started them. Map keys arrive in _func_godot_apply_properties. Read them through GodotTrenchIO.to_bool, to_vector3 and to_color, which accept both text and already converted values.

Making it placeable

Add a definition for it to your project's FGD with the script as its script class, see Project setup. The game config then exports it to the editor, with its signals as outputs and its methods not starting with _ as inputs. The Reference panel's Create in project writes a class skeleton for any entity.

Calling I/O from code

Use the same functions the map uses:

for door in GodotTrenchIO.find_targets(self, "main_door", null):
    GodotTrenchIO.invoke(door, &"open", "", null)

Go through invoke rather than calling the method directly. It fits the parameter to the method's arguments the way map connections do, and handles the built-in inputs.

To fire every connection on one of an entity's outputs by hand, use GodotTrenchIO.fire_output(source, &"tripped", activator).