I finally got my Windows 10 working tonight, so I can go through the book.
I got this same error, and found the solution was to add “-c conda-forge” to the command above. I guess this means the “gh” package is in the conda-forge channel? It worked.
Installers for “gh” is not available for Windows as can be seen from the anaconda repo . We can Install fastai first and then try to install 'gh" separately with conda-forge.