Well, `wheel` allows you to run `su`, so I don't think it's so bad if being in that group also allows you to run `sudo`...

I face the same problem in my Arch port, and I implemented it with something like:

    if [ ! -e /etc/sudoers.d/00-wheel ]; then
        echo '%wheel ALL=(ALL) ALL' > /etc/sudoers.d/00-wheel
    fi

Provided that Gentoo also sources `/etc/sudoers.d/*` files. It's also a nice solution as it allows the user to manually disable it if wanted (just comment out the line in `/etc/sudoers.d/00-wheel` and it won't get updated)