When GRUU is generated by SCSCF and used by UE, the SBC transparently passes the received parameters in 2xx response of REGISTER to the UE if passCompleteContactHeader transparency flag is enabled on both Ingress and Egress trunk group (see commonIpAttributes Common IP Attributes - SIP - CLI).
For dialog creating INVITE/SUBSCRIBE or OOD request, the SBC is able to transparently pass the GRUU received in the contact header from UE to the core.
OOD dialog REFER is relayed to the core using Stored/Received route set. Hence no special processing is done at SBC (Refer-To containing GRUU is transparently passed since Refer is being Relayed)