The replication gateway must communicate with one ODSEE 11gR2 master. However the rest of the topology does not need to be upgraded to 11g.
Many customers deploy the replication gateway and the ODSEE11g instance on the same box; The ODSEE11g can be seen as a companion of the replication gateway.
Note that some specific limitations apply when DSEE5.2 is in the picture: Some Password policy related state, e.g. nb of failed authentication are not fully replicated between DSEE 5.2 and OUD.