How do I find all files that are *not* group writeable?

Assuming that the linked question would run on your system, you can invert the match with !:

find ! -perm -g=w

Also, you can read the documentation for find by looking into the manpages:

man find

On BSD derived Unix system (macOS, FreeBSD, ...) you need to add a path to find. So for a more compatible version add . to search in the current directory and ! to invert the match:

find . ! -perm -g=w