I don't have much detailed knowledge in MBMB area but aware about little bit.
MME and MBMS-GW has GTPv2 based interface i.e. known as Sm interface. This interface is used only for signalling message not for data, similar to lte S11 interface.
There are two types of interface between MBMS-GW and BMSC. One interface SG-mb , used for signalling messages exchange between MBMS-GW and BM-SC nodes. There is another interface SGi-mb between BMSC and MBMB-GW, used for data plane.
Once the necessary signalling completes between BMSC and rest of the required nodes of network, user data flows from BMSC to MBMB-SGW and the gateway multicast the received IP packets towards E-UTRAN (eNodeBs).