If you found the 3GPP Gx tutorial interesting (here), you probably want to learn also about its companion, Gy. Bart Barton explains Gy in his "LTE and Beyond" blog.
"Gy is between Online Charging System (OCS) and PCEF. In most cases PCEF is based inside PDN GW (Packed Data Network Gateway) or just short PGW. Gy interface allows online credit control for service data flow based charging"
See "Gy interface - sitting between OCS and PCEF" - here.