Developer reply by Philippe V.
Rated 2 out of 5 stars
I have tried this extension with Gmail, first time it works but only with contacts that are in the group that my android phone displays as "My Contacts" (I guess this is the default group in Gmail when you create/modify a contact). They are in Cardbook in a category named "Uncategorised".
I delete this category by mistake, the add-on doesn't work anymore (I see the account but no more synchronization). It seems reinstalling does not solve the issue (even by deleting manually the Cardbook items in the profile directory), I guess some residual items are kept somewhere.
So this add-on seems promising but the last update may have some bugs, I will definitively retry again when all contacts in all gmail groups could be loaded (if it's possible).
Google is a nightmare because it has not a proper implementation of CARDDAV : categories are not synchronized, ids are changed, etc...I will concentrate my efforts for the moment for systems which respect open standards and in future maybe I'll try to make something for them... Try Owncloud, sorry ;O)
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