Enable option to make GF fields readonly

gf-readonly-fields.php

<?php

add_filter('gform_field_content', function ($field_content, $field) {

	$fieldtype = $field['type'];
	if( $fieldtype == 'text' || $fieldtype == 'email' || $fieldtype == 'number' || $fieldtype == 'phone' ) {
		$readonlySupport = 'true';
	} else {
		$readonlySupport = 'false';
	}
	$visibility = $field['visibility'];

	if ($visibility == 'readonly' && $readonlySupport == 'true' ) {
		$field_content = str_replace('type=', "readonly='readonly' type=", $field_content);
	}

	elseif ($visibility == 'readonly' &&  $field['type'] == 'textarea' ) {
		$field_content = str_replace('name=', "readonly name=", $field_content);
	}

	elseif ($visibility == 'readonly' &&  $field['type'] == 'checkbox' ) {
		$field_content = str_replace('type=', "disabled='disabled' type=", $field_content);
	}

	elseif ($visibility == 'readonly' &&  $field['type'] == 'radio' ) {
		$field_content = str_replace('type=', "disabled='disabled' type=", $field_content);
	}

	return $field_content;
}, 10, 2);

add_filter( 'gform_visibility_options', 'wcd_gf_readonly_visibility_options' );
function wcd_gf_readonly_visibility_options( $options ) {

    $options[] = array(
        'label'       => __( 'Read Only', 'wcd' ),
        'value'       => 'readonly',
        'description' => __( 'Set the field to read only so value cannot be edited.', 'wcd' )
    );
    return $options;
}