Ro is a 3GPP reference point that describes the connection to the OCS from another functional component. It is a general term that covers multiple connecting components such as GGSN, P-GW, WLAN, IMS AS, MBMS server, etc. See 3GPP TS 32.296  Figure 5.1 for reference.
Gy is an specific instance of the Ro reference point that connects a charging component from the packet-switched domain (PS domain) i.e. PCEF within a GGSN or P-GW to the OCS. See 3GPP TS 32.251  Figure 4.3-1 for reference.