GT – is world wide identificator of network node and used in inter-PLMN signalling interworking (e.g. international roaming).
DPC – is significant only in given MTP network and used in interworking within PLMN and also as GT translation result.
SSN – is for message distribution within network element (MSC-MAP, VLR-MAP, HLR-MAP etc.)
Let me use the analogy with the Internet Protocol.
Originating Point Code (OPC) and Destination Point Code (DPC) is similar to the Source and Destination IP address. With SS7 (or C7 (BTNR 167)) you have a stack that is layered: MTP/SCCP/TCAP/Application (MAP/INAP/CAP(CAMEL)etc..).
IP uses Application (e.g. SMTP, FTP, HTTP, DNS, etc..) / TCP / IP / Ethernet.
OPC and DPC is used by the lower MTP – Message Transfer Part.
GT – Global Title – defines a signalling point (node) in the Network.
SSN – Sub-System Number – is similar to a port that is used with TCP/IP.
A socket() is an IP address plus a port. The port number will point to the application that
will use the services of TCP/IP. Example is port 80 points to HTTP. Port 20/21 (Data / Control)
points to FTP. SSN points to different applications, e.g. EIR, HLR, MSC, IN, etc..