You can now send SMS messages to your contacts mobile phones using Gmail Chat but to use this feature you need to enable the SMS (text messaging) in Chat lab ( you can access chat tab from mail setting )

After enabling Sms ,Enter your contact’s name in the ‘Search or invite friends’ box in Chat, and 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.

In the dialog box, enter a phone number in the ‘Send SMS messages to this number’ field.If you dont see this option then you need to enable the SMS feature from chat tab.

Click Save.

A Chat window appears. Just type your message as you would normally. When you hit Enter, the message will be sent to the phone number you entered.

Supported mobile operators for SMS Chat in Gmail.

Click here to check whether your countries and operators are supported by gmail or not for Sending free sms from gmail.