I cannot see how I could authenticate a server with vs GAE.
Let's say I have an application on GAE which have some data and I somehow need this data on another server.
It is easy to enable OAuth authentication on GAE but here I cannt use this since there is no "account" binded to my server.
Plus GAE doesn't support client certificate.
I could generate a token for each server that needs to access the GAE Application, and transfe them on the server. It would then use it to access the GAE Application by adding it in the URL (using HTTPS)...
Any other idea?
That is exactly what you need to do. On the server, generate a key (you choose the length), and store it in the datastore. When the other server makes a request, use HTTPS and include the key. Its like an API key (it is actually).