This is a very confusing question.
If you have a router connecting with your ISP which would forward your traffic to a desired destination, that would be a next-hop "gateway".
So for example, if you have on R1
Ethernet0
Ip address 10.10.10.1 255.255.255.252
!
interface vlan1
ip address 192.168.30.1 255.255.255.0
!
ip route 192.168.30.0 255.255.255.0 10.10.10.2
your ISP should have another interface against you with
interface X
ip address 10.10.10.2 255.255.255.252
Then it depends on where the traffic from 192.168.30.0/24 should end.
If you have a computer behind vlan1 on R1, the default-gateway for that computer should be 192.168.30.1 (vlan1) your router R1 would then forward that traffic to your ISP and your ISP would forward that traffic to a desired location.
This is a normal setup for branches. If you have a pure internet connection, you would forward your traffic to the outside interface (ATM0/ETH0/DIALER0..ETC)
Based on the confusion regarding this question I have no idea if that satisfies your needs, if not please provide a more detailed question.
thanks