wsl-gap is a gpg-agent proxy that relays communication between stdin/stdout and the gpg-agent running on Windows.
Together with socat in WSL it is used to forward a socket from WSL to the gpg-agent running on Windows to be able to access the agents keys from WSL. This makes it possible to use keys from a hardware dongle, like a Yubikey, or a smartcard, which are not directly accessible inside WSL.

wsl-gap can be used for both gpg and ssh and works in both WSL1 and WSL2.

Releases can be verified using:

minisign -Vm wsl-gap.exe -P RWQnxc7i4M9iJ5oE7m4ry4dchugn3fgsHHdbUHcJOMvf/KSO+t7fxQ/v

$ wsl-gap.exe -h
wsl-gap 1.0.0

USAGE:
    wsl-gap.exe [FLAGS] [socket-path]

FLAGS:
    -d, --debug      Activate debug logging to file in the same directory as the exe
        --gpg        Run as gpg-agent proxy
    -h, --help       Prints help information
        --ssh        Run as ssh-pageant proxy
        --version    Show version information

ARGS:
    <socket-path>    Path to a socket on the Windows side if not using the default

• The gpg-agent is started if not already running.
• The GNUPGHOME environment variable (if set) or AppData\Roaming\gnupg will be used as path to the S.gpg-agent socket on the Windows side.
• The GPG and/or SSH sockets are created in /tmp.
  • gpg v2.1+ expects the S.gpg-agent socket to be located in GNUPGHOME, so it must be forwarded by creating the file $GNUPGHOME/S.gpg-agent with the following content:

    %Assuan%
    socket=/tmp/S.gpg-agent
    

• GnuPG installed and working in Windows, with the path to gnupg\bin added to the Windows Path environment variable.
• socat installed in WSL.
• wsl-gap.exe accessible and executable from WSL.

To only setup for ssh, just export SSH_AUTH_SOCK and use socat to relay between the socket and wsl-gap.exe with the --ssh argument.

export SSH_AUTH_SOCK=/tmp/S.gpg-agent.ssh

if [ ! -f "$SSH_AUTH_SOCK" ]; then
    (setsid socat UNIX-LISTEN:$SSH_AUTH_SOCK,fork EXEC:"/absolute/path/to/wsl-gap.exe --ssh" &) >/dev/null 2>&1
fi

To use for gpg, configure gpg on the WSL side, create the S.gpg-agent file to redirect to the actual socket, and use socat to relay between the socket and wsl-gap.exe with the --gpg argument.
Then import the public keys and it should work.
Note! The GNUPGHOME directory cannot be the same on the WSL side as on the Windows side. Using the default home directory (/home/user) in WSL is a safe way to avoid this.

export GNUPGHOME=/home/user/.gnupg
GPG_AGENT_SOCK=/tmp/S.gpg-agent

# Create S.gpg-agent redirect file if it doesn't exist.
if [ ! -f "$GNUPGHOME/S.gpg-agent" ]; then
    echo -e "%Assuan%\nsocket=$GPG_AGENT_SOCK" > $GNUPGHOME/S.gpg-agent
fi

if [ ! -f "$GPG_AGENT_SOCK" ]; then
    (setsid socat UNIX-LISTEN:$GPG_AGENT_SOCK,fork EXEC:"/absolute/path/to/wsl-gap.exe --gpg" &) >/dev/null 2>&1
fi

See the script wsl-gap.sh for an example on how to setup. The script can be included in your .bashrc and used to setup for both gpg and ssh.

https://github.com/benpye/wsl-ssh-pageant and https://github.com/BlackReloaded/wsl2-ssh-pageant for inspiration.
https://github.com/Yasushi/putty/ for showing how to communicate with Pageant.