Yes, yes they can. What would ever make you think otherwise?

Not even when "Don't be evil" was still their motto did Google not only have the ability to read all communication that was routed through them, they actively used those messages to develop their offerings.

The only system or organization you should trust to not read your messages is one that never gets to see your messages unencrypted. In all other cases, assume the organizations involved in relaying your message can read them, and will do so if they (or any of their employees, or contractors, who happen to have access) think of a reason.