How to add option groups in the Gravity Forms select field

There currently isn’t built-in support for option groups in Gravity Forms.

In the meantime it is possible to use the gform_field_choice_markup_pre_render filter to change some choices into opening and closing optgroup tags:

On the field in the form editor configure the choices like so:

label | value
===============
group one | optgroup-start
choice one | choice 1
choice two | choice 2
choice three | choice 3
end group one | optgroup-end
group two | optgroup-start
choice four | choice 4
choice five | choice 5
choice six | choice 6
end group two | optgroup-end

When the field is being displayed the code snippet will handle replacing the HTML for those choices with the optgroup-start and optgroup-end values with the correct option group HTML tags.