Gx and Gy are different Diameter "Applications". Their PDUs different. Though they use the same Diameter commands (CCR/CCA, RAR/RAA), similarity ends there.
Gx is used for policy control. PCEF sends request for Policies and PCRF responds with Policies.
Gy is used for billing and related authorization etc. PCEF or some other function sends the subscriber information and Onile Charging System responds with authorization (for the usage of resources) and any additional information like how long the service can be consumed, when to report back etc.
So, apart from the fact that they are Diameter extensions (and thus connection setup, request forwarding, session management etc are done in the same way), they don't have much in common. Of course, they have many AVPs in common.