Add the optgroup ability to Gravity Forms default select field.

filter_gf_select_optgroup.php

/**
 * Filter Gravity Forms select field display to wrap optgroups where defined
 * USE:
 * set the value of the select option to `optgroup` within the form editor. The 
 * filter will then automagically wrap the options following until the start of 
 * the next option group
 */

add_filter( 'gform_field_content', 'filter_gf_select_optgroup', 10, 2 );
function filter_gf_select_optgroup( $input, $field ) {
    if ( $field->type == 'select' ) {
        $opt_placeholder_regex = strpos($input,'gf_placeholder') === false ? '' : "<\s*?option.*?class='gf_placeholder'>[^<>]+<\/option\b[^>]*>";
        $opt_regex = "/<\s*?select\b[^>]*>" . $opt_placeholder_regex . "(.*?)<\/select\b[^>]*>/i";
        $opt_group_regex = "/<\s*?option\s*?value='optgroup\b[^>]*>([^<>]+)<\/option\b[^>]*>/i";

        preg_match($opt_regex, $input, $opt_values);
        $split_options = preg_split($opt_group_regex, $opt_values[1]);
        $optgroup_found = count($split_options) > 1;

        // sometimes first item in the split is blank
        if( strlen($split_options[0]) < 1 ){
            unset($split_options[0]);
            $split_options = array_values( $split_options );
        }

        if( $optgroup_found ){
            $fixed_options = '';
            preg_match_all($opt_group_regex, $opt_values[1], $opt_group_match);
            if( count($opt_group_match) > 1 ){
                foreach( $split_options as $index => $option ){
                    $fixed_options .= "<optgroup label='" . $opt_group_match[1][$index] . "'>" . $option . '</optgroup>';
                }
            }
            $input = str_replace($opt_values[1], $fixed_options, $input);
        }
    }

    return $input;
}

@SeanDKendle Yes it is
https://docs.gravityforms.com/select-field/

Thanks for this! Had to change the regex to expect a space after value='[value]', and I changed it so the "optgroup" was the label instead of the value, since the values were coming from an array in PHP, and it just replaced the optgroup item over and over and left me with one optgroup. Otherwise, this was a huge help!

<\s*?option\s*?value='([^<>]+)'\s*?>optgroup<\/option\b[^>]*>