This is straightforwardly false: the user does not send anything over Google servers in the course of using Signal. Signal sends an empty push notification over GCM to users who have received a message; while technically true, to, in a discussion of a messaging platform, refer to empty push notifications as sending "messages over Google servers" is prone to misinterpretation as false statements that the "messages" going over Google servers contain any information about the messages being sent over the messaging platform.
Additionally, the use of GCM only allows Google to extract metadata about when a user receives messages; GCM messages aren't necessary to notify you when you've sent a message, so I'm not sure how you propose that Google can infer when a user sends a message and who that user is messaging. Could you elaborate on this?
Signal is seemingly built around providing the image of anonymity and security, but has glaring design flaws which negate these.