TS 29.274
GTP-PDU: GTP Protocol Data Unit is either a GTP-C Message or a GTP-U Message. GTP-U Message may be either a signalling message across the user plane tunnel, or a G-PDU (see clause 6).
• Signalling Message: any GTP-PDU (GTP-C or GTP-U) except the G-PDU.
• G-PDU: GTP user plane message, which carries the original packet (payload). G-PDU consists of GTP-U header and a T-PDU.
GTP-U Message: GTP user plane message. The user plane messages are used to carry user data packets, and also signalling messages e.g. for path management and error indication. Therefore, GTP-U message consists of GTP-U header, which is followed by either a T-PDU, or zero or more information elements.