Option groups in the Gravity Forms drop down field

gravity-forms-option-groups.php

<?php
// Do NOT include the opening php tag

add_filter( 'gform_field_choice_markup_pre_render', function ( $choice_markup, $choice, $field ) {
	if ( $field->get_input_type() == 'select' ) {
		$choice_value = rgar( $choice, 'value' );
		if ( $choice_value === 'optgroup-start' ) {
			return sprintf( '<optgroup label="%s">', esc_html( $choice['text'] ) );
		} elseif ( $choice_value === 'optgroup-end' ) {
			return '</optgroup>';
		}
	}

	return $choice_markup;
}, 10, 3 );