From OpenPGP specification (RFC 4880) section 13.8 (Reserved Algorithm Numbers):
The reserved public-key algorithms, Elliptic Curve (18), ECDSA (19),
and X9.42 (21), do not have the necessary parameters, parameter
order, or semantics defined.
But GnuPG has ECC support. What they use as a reference? How other developers could support ECC and read messages from GnuPG with ECC?