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.