Gb interface can use FR or IP transport mode, but the protocol is still the same :
TS 48.018 BSS – SGSN interface ; BSS GPRS protocol (aka “BSSGP”)
In other words, the “BSSGP” messages sent between PCU and SGSN are the same, they are just “transported” differently.
With IP, what you get is a BSSGP message within an IP packet.
With FR, the BSSGP message is transported within a frame relay PDU.