I am developing a desktop application in VS2008 with C#. How can I send an SMS using a secure gateway?
There isn't any common example you can get.
First you have to contact a SMS gateway provider (for example your mobile phone provider) and request that you want to send SMS messages via their gateway. They will provide you with all the tehnical information and most of the time there is even examples for most of the platforms (ASP.NET, PHP, etc).
Just get the string from an sms provider gateway and make the simple web request from your program. Remember to replace the text and the mobile no value before making web request.