This feature is only available in Inspector Gadgets Pro.
If you require more customisation than you can achieve using the available Decorator Attributes, Inspectable Attributes, and Event Messages, you will need to create your own custom editor class. You can do this quickly using Ctrl + Middle Click in the target script's Inspector or using the Create Editor Script context menu function.
By inheriting from InspectorGadgets.Editor<T> instead of UnityEditor.Editor your custom editor will inherit the features described in previous sections (hide Script field, Middle Click to open script, support for Inspectable Attributes and Event Messages). You will also be able to access the inspected objects directly using the Target and Targets properties (note the capitalisation) rather than needing to type-cast the regular target and targets properties yourself.