how to use mapproxy to serve WMS from reprojected google maps tiles, i have searched for docs, but there's no docs which describe how to add google maps as source.
Technically it is very simple to use Google tiles as source for WMS. You can do this with the following MapProxy config:
But as mentioned above - you are allowed to access the tiles only through Google API.
MapProxy supports WMS sources (1.0.0–1.3.0)& TMS/WMTS sources.
Google does not have an WMS service. It has its own tiles, and according to Google's Terms & Conditions, you are allowed to access the tiles only through their API.
This is why you cannot use MapProxy with Google Maps as a source.