If the gateway is included in a Route Group the drop-down list box does not display the the individual gateway.
The gateway will also not apppear if you have already assigned the gateway to a route-group, but you have not assigned the route-group to a route-list.
If you have created a gateway, it should appear in the drop box, unless of course you have assigned it to a route-group already.
Similarly if you have assigned a gateway to a route-pattern, then you will not be able to add that same gateway to a route-group. Unless you delete the route-pattern first.
HTH
Allan.