If in case the SMS Gateway allows sending SMSes by a accessing a simple url with authentication etc params ( example:, http://<some third party sms gateway [smsala.com]>/index.php?username=john@smith.com&password=randompassword&to=9876543210&message=testmessage)
then this all you need is
- a simple html form
- which can post the message
- to a php script which can make a CURL call to the SMS Gateway URL
sample code for a CURL call