You can NOT use Stealth with GPG 'as it', here is why and around :
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http://lists.gnupg.org/pipermail/gnupg-users/2001-July/009338.html

You can probably use Stealth with gpg if you generate pgp 2.6.x compatible
messages (gpg --rfc1991 --cipher-algo idea --no-literal) using an old
style v3 RSA key.

Er, make that "gpg --rfc1991 --cipher-algo idea --compress-algo 1".
Wrong cut-n-paste :)

[idea is a plugin]

You might want to also look into gpg --throw-keyid.  While that does
not result in a message that resembles random noise (it's still
clearly a openpgp message), it does remove the ID of the user the
message was encrypted to which provides some level of security for a
message intended for steganographic hiding.

Oh, it's certainly possible.  You might even be able to adapt the
existing stealth code for openpgp messages.  The hard bit is that
openpgp allows for different symmetric and asymmetric ciphers among
other things - part of what made stealth as simple as it is was the
guarantee that every message would be RSA/IDEA.