What you are asking is impossible. An SMS gateway, by definition, is a gateway between SMS and TCP/IP. It allows SMS messages to be sent via TCP/IP. If you have no Internet connectivity in the field locations, then it isn't possible to use an SMS gateway. You just need to use the SMS on the device.
Access to SMS on the device varies by platform. On iOS, for example, you can prepare a message and send it to the Messages app, but then the user will have to push the "Send" button complete the transmission. On other platforms, it may be possible to access SMS directly.
Yes, Jon, I realized that to use the gateway, a Net connection would be required, whether its for an app running on the device or for a Web app.
What do you suggest for such scenarios, is storing the data on device in offline mode and then syncing to database server whenever online the only way?