Hello, Bassem!
The Gr-interface in GPRS is analogue of D-interface in GSM. It is used to exchange the data related to the MS location and to the management of the subscriber.
There are several procedures on it when e.g. GPRS Attach in new SGSN area or Inter-SGSN Routeing Area Updating is performing (see 23.060, 29.002 for more information):
1)Update Location for GPRS (SGSN number,SGSN address, IMSI) from SGSN to HLR;
2)Insert Subscriber Data for GPRS ((IMSI, GPRS subscription data) from HLR to SGSN;
3)Insert Subscriber Data for GPRS Acknowledge (IMSI) from SGSN to HLR;
4)Update Location for GPRS Acknowledge from HLR to SGSN.