Here is a step-by-step procedure how to send SMSes from one's Gmail account.
In order to use this feature you have to enable the SMS (text messaging) in Chat lab which is located in the chat setting menu.
Open your Gmail account.
Locate the Chat section. If the recipient's name is already in your Chat list, then just enter your contact's name in the 'Search or invite friends' box in Chat. If his mobile number is not there, then enter it manually to send the SMS.
After that, select Send SMS from the box of options that appears to the right of your contact's name. Or, if you already have a Chat window open for this contact, just click Options, and select Send SMS.
If the person is not enlisted in your Gmail chat account, then in the dialog box, enter his phone number in the 'Send SMS messages to this number' field. This feature is only launched in selected countries including India where all mobile operators are part of it.
Just type your message as you would normally. When you hit Enter, the message will be sent to the phone number you entered.
If your contact replies, the text message response will appear as a reply in Chat. These conversations are stored in your Chat history just like regular chats (but keep in mind that you can't go off the record while communicating via SMS).