I’m not sure that just copying the code from the gallery field will work. You’d need to work that code into the class the developer has provided for creating custom field types. A lot of the core fields do not work the same way that class functions.
If I were doing this I would use a repeater field for this rather than a gallery field. But then I’m not sure I completely understand what you’re doing. You might also be able to do this using a checkbox field where the labels for the fields are the images so that they can be selected.
