Gb is between the PCU (MFS) and the SGSN, carrying GCH and GSL. This link is traditionally carried thanks to Frame Relay links, and nowadays, it can also be carried thanks to IP links.
OSS is the link for supervision (link to OMCR), it can also be carried by IP link. It usually connects the BSC and PCU to the OMCR.
Cheers,
Pix