Veja esta página para a dll e como instalá-la
As instruções são as seguintes:
Installing iconv.dll
To support different languages and character sets, Microsoft Windows versions of GnuPG make use of a DLL called "iconv.dll". This DLL has been written by the GNU project too and is used by several applications ported from GNU/Linux to MS Windows systems. If this library has not yet been installed on your system, an error message might have you taken to this page. Please follow the instructions below to install this library.
GnuPG has a fallback mechanism to work even without this DLL. However some texts may not show up correclty and when creating a new key it might not be encoded correctly. Thus unless you are using the Latin-1 or UTF-8 character set, you should definitley install the DLL before you create a new key or add a new user ID to a key.
If you have used the Gpgwin package to install GnupG, you should never see this message because this DLL will get installed. However, it might happen that such a DLL has been insatlled on your system by another application and later be deinstalled. To rectify this, simply re-install Gpg4win and your are done.
The DLL is available as a ZIP file at the address ftp://ftp.gnupg.org/gcrypt/binary/libiconv-1.9.1.dll.zip. You should also get the corresponding ".sig" file and verify the integrity . This DLL has been built from the source package libiconv-1.9.1.tar.gz available at the same FTP sites. We distribute it separately for convenience and because GnuPG only requires this DLL and not the other files from the binary package. For copying conditions see the included file "COPYING.LIB".
To install this DLL, unzip it and copy the file "iconv.dll" to a directory where you usually keep DLLs. If you don't know such a directory, it is safe to copy it into the same directory as the "gpg.exe" file is stored.