GSUP-to-DIAMETER converter / IWF
In order to support a single subscriber database for 2G/3G and LTE, operators normally use a single HSS. HSS is the EPC successor of the LTE.
Instead of MAP over SS7, HSS's speak DIAMETER over SCTP or TCP (with optional TLS). The types of procedures / transactions are pretty much the same as before, but the encoding and the protocol changed completely.
There are a couple of (at least minimal) HSS implementations out there, from freeDiameter to nextepc, to name some examples.
If we were to implement a converter between GSUP and DIAMETER, then the OsmoMSC and OsmoSGSN could access an external HSS - and that same HSS could be accessed from nextepc or even openair-cn for LTE access.
3GPP TS 29.305 specifies the MAP-to-DIAMETeR InterWorkingFunction (IWF), which is pretty much the same device, with the exception that we'd use GSUP instead of MAP.