GizmoRendererAttribute.cs

using System;
using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using UnityEditor;

public class GizmoRendererAttribute : System.Attribute
{
	private Type classWithGizmos;

	public GizmoRendererAttribute(Type t)
	{
		classWithGizmos = t;
		if( ! classWithGizmos.IsSubclassOf(typeof(MonoBehaviour)))
			throw new Exception( "This attribute must specify a target MonoBehaviour that it will render gizmos for");
	}

	public bool isRendererForClass(MonoBehaviour mb)
	{
		return mb.GetType().IsAssignableFrom(classWithGizmos);
	}

	public static Dictionary<GizmoRendererAttribute,MethodInfo> gizmoCache;

	public static List<MethodInfo> RenderersForObject( MonoBehaviour mb )
	{
		if(gizmoCache == null)
		{
			gizmoCache = new Dictionary<GizmoRendererAttribute, MethodInfo>();
			
			var assembly = AppDomain.CurrentDomain.GetAssemblies();
			var gizmoRendererMethods = assembly.SelectMany( x => x.GetTypes() )
				.SelectMany(type => type.GetMethods())
				.Where(type => Attribute.IsDefined(type, typeof(GizmoRendererAttribute)) );

			foreach( var method in gizmoRendererMethods )
				if(Attribute.GetCustomAttribute(method, typeof(GizmoRendererAttribute)) is GizmoRendererAttribute attribute)
					gizmoCache[attribute] = method;
		}

		return gizmoCache.Where(pair => pair.Key.isRendererForClass(mb)).Select(pair => pair.Value).ToList();
	}
}

GizmoRendererSimpleExample.cs

public static class GizmoRendererSimpleExample
	{
		[GizmoRenderer(typeof(MyMonoBehaviour))]
		public static void OnDrawGizmos( MonoBehaviour source )
		{
			Gizmos.color = Color.magenta;
			Gizmos.DrawCube(source.transform.position, Vector3.one);
		}
	}

MyMonoBehaviour.cs

public class MyMonoBehaviour : MonoBehaviour
{
 void OnDrawGizmos()
 {
			GizmoRendererAttribute.RenderersForObject(this).ForEach(info => info.Invoke(null,new []{this}));
 }
}