Google is your friend: Be nice and visit him often. He can answer questions a lot more quickly than posting them here...
A very quick search using part of your subject as the search term
gave : http://www.google.co.uk/search?sourceid=chrome&ie=UTF-8&q=free+sms+gateway+api
Just one of the links there was this: Free SMS gateway / API.
So, if you have been searching for two days, you need to brush up your google-fu!
In future, please try to do at least basic research yourself, and not waste your time or ours.