Gmail SMS was an experimental SMS feature that Google launched in 2008 and discontinued on April 1, 2014. Prior to its discontinuation, Gmail users were able to send SMS messages to contacts on supported mobile operators through the chat feature in Gmail.
Within the Google product ecosystem, there are other ways to send SMS messages, including an optional feature on Hangouts in which messages can be received through SMS. However, the SMS feature on Hangouts does not have the same international coverage as Gmail SMS. For Google users in the U.S. or Canada, it is still possible to send SMS messages through Google Voice as of 2014.