Your gateway IP address is going to be the starting IP address of the subnet assigned to you.
For example:
Assigned Subnet - 192.168.200.128/25
Useable IP address range - 192.168.200.129-254
Gateway IP address - 192.168.200.129 (which is hosted on our router)
Customer Range - 192.168.200.130-254
If you have what we call a serial connection or slash 30 address your gateway will be the first useable IP address in the slash 30 space.
For example:
Assigned Serial Subnet - 192.168.200.0/30
Assigned Subnet - 192.168.200.128/25
Useable IP address range - 192.168.200.1-2
Gateway IP address - 192.168.200.1 (which is hosted on our router)
Customer Range - 192.168.200.2, 192.168.200.129-254