What tools are available to edit manpages, i.e. GUI WYSIWYG editors (any platform)
You want ronn.
Ronn builds manuals. It converts simple, human readable textfiles to roff for terminal display, and also to HTML for the web.
It's not WYSIWYG, but it does let you export normal text files. Since nroff doesn't support arbitrarily positioned text that's about as good as it gets.
There is an online editor for man pages in mdoc format: http://roperzh.github.io/grapse/
Mdoc is a semantic markup for man pages. It has been around for about two decades and supported by Linux and other Unix-like OSes. You should prefer mdoc to troff.