OpenPGP as defined by RFC 4880 knows two different encodings.
Obviously, there is no reasonable limitation to an (ASCII) character subset in binary encoding.
Radix 64 is also often called ASCII armored. In the end, it is a base64 encoding with a checksum. The content may consist of
ASCII-armored OpenPGP messages also include a header, which may contain arbitrary UTF-8 strings.
An example message in ASCII-armor (modified from the one in the RFC):
-----BEGIN PGP MESSAGE-----
Version: OpenPrivacy 0.99
Comment: An example for an UTF-8 string with arbitrary characters
-----END PGP MESSAGE-----