Depends on the size of the system. In the design we're about to move forward with we have 3 trunking gateways in a ring configuration. As kwbMitel suggests, the ars for the non-gateway machine is simple. Route it all to the gateways in a route list.
Our decision to use 3 gateways instead of 2 is so we can take one down at a time for upgrades, or in the event we lose one it doesn't create a crisis (or TDM trunk blocking). Triple redundancy is there for a reason and if you ever need it you'll better understand why we're doing it. The cost of the 3rd controller is really incremental and only a very few more licenses. You'll absolutely want to use SDS.
And now instead of crowding all of our TDM trunks into 2 controllers and potentially losing 50% of external call capacity, the risk is reduced to 33%. In such a configuration we could lose 1 gateway and in all but the most busiest peak time of the business day our users wouldn't even know it's down. If you crowd a busy system into just 2 gateways and lose 1, your users will feel the pain.
Unless you're one of those IP Purists and have deep pockets, your gateways are also a good place to connect your Analogs, perhaps even use a dual fim and an old 2K per node or two since that way you don't need to buy any analog device licenses or buy any SIP analog terminal adapters & SIP licenses. Just IMO licensing analog devices (modems, faxes, Polycoms, etc) when I've already felt the pain of buying 1400 IP licenses is borderline ludicrous. The old 2K PER nodes are plentiful, will practically run forever and MC320 ONS cards are dirt cheap on the secondary market. Keeping a PER node alive also gives you some very unique opportunities for non-Kosher tricks with trunking and paging when the client comes to you with a bizarre request.