Layer Attribute for Unity Inspector (from https://answers.unity.com/questions/609385/type-for-layer-selection.html)

LayerAttribute.cs

using UnityEngine;

/// <summary>
/// Attribute to select a single layer.
/// </summary>
public class LayerAttribute : PropertyAttribute
{
}

LayerAttributeDrawer.cs

using UnityEditor;
using UnityEngine;

[ CustomPropertyDrawer( typeof( LayerAttribute ) ) ]
public class LayerAttributeDrawer : PropertyDrawer
{
    public override void OnGUI( Rect position, SerializedProperty property, GUIContent label )
    {
        property.intValue = EditorGUI.LayerField( position, label, property.intValue );
    }
}