I just re-ran the command and its working now. I'm guessing it was temporarily offline. I was able to browse to https://registry.npmjs.org/@microsoft/microsoft-graph-client but when I tried browsing to https://registry.npmjs.org/@microsoft/generator-sharepoint I got a "not found" error. Browsing the URL is also working now.
You should either fix all warnings or if it's not possible, you can ignore them.
For that purpose, you should modify your gulpfile.js.
There is one out of the box build suppression method call in gulpfile. You can add your own one using the same build.addSuppression method.
The method also accepts regexp, which is very convenient for us. Just add
For anyone else struggling to find the .gcb-serve-data folder, an example path is...
C:/Users/PCOX/.gcb-serve-data i.e. the same route as wehre the documents, music and downloads libraries are, its also a hidden folder.