Does a gateway need to be designated?

I thought a gateway between two networks is a device explicitly designated to be a gateway, out of several devices connected to both networks. That was my impression when I heard that I could find out gateways from a routing table (e.g. from the output of route -n).

But now suppose that I am to create a new network, and add a line to the routing table of a host, so I have to pick a gateway, without looking up the routing table.

Is a gateway between two networks just any (meaning without being explicitly designated) device

• with two network interfaces connected to the networks respectively, and

• with a routing table allowing packets to be transmitted from a network to the other (Is it correct that every device in network(s) has a routing table?) ?

Thanks.