In many cases you would want to test your applications with serveral types of gateways, just because they work in different ways. You might also want to connect to another gateway than the one your operator provides for some reason or other.
   
WAPdrive.net has a very good list of public gateways available at http://www.wapdrive.net/DOCS/wap_gateways.html. Note that the list includes gateways that are not publically available. If the IP address starts with "10.", then the gateway is private and only reachable through the operator/owner. Gateways with IP addresses in another range does not guarantee public availability. (Also note that the site was very slow when I tested it)
 
How to change your gateway setting should be explained in your WAP device's user manual, or you can have a look here.