It can be said that it is different, and it can be said that there is no difference.
A gateway is an exit (router) to another network. The default gateway is the gateway used when the route is unknown (default route).
In terms of server-side settings, specifying a gateway is usually only the default gateway, so specifying a gateway is often synonymous with specifying a default gateway (should).
There is also an OS that specifies as an alternative path to switch when a failure occurs in a gateway that has multiple default gateways.