The search engine initially launched the function in Gmail labs in October this year, but quickly removed it after a glitch was discovered. However the glitch has now been removed and the function has been reintroduced.
The service only works for US-based mobile numbers but allows Gmail users to chat via instant messaging with people who are away from their PCs. Users can either let their mouse hover over a contact in Gmail Chat and then click on the 'Video & More' option to select SMS.
Alternatively, use the Options menu to switch from an open chat window to SMS, or type the mobile number into the Chat search box and select 'Send SMS'.
The person receiving an SMS from Gmail's Chat would see it as coming from a Google-generated mobile phone number. The handset owner then simply sends a text message back to this number and it will be delivered straight to your Chat window.
This way of communication will remain until you manually switch back to regular chat or close the chat window. This is important because the service isn't free.
The person receiving SMS messages from you via Gmail Chat will be charged by their mobile carrier each time you hit enter in the chat window. The same applies if they reply to that message. While sending the messages from Gmail is free, at the receiving end users will pay standard text messaging rate.
If you find you are on the receiving end of too many Gmail texts, simply reply to Gmail SMS with 'BLOCK' and the Gmail user will be prevented from sending you messages. Replying 'STOP' to a Gmail SMS blocks all Gmail-initiated text messages.
Google doesn't say whether you can elect to start receiving Google SMS messages again so you might want to think carefully before using the 'STOP' option.
For the moment, Google says you can only send texts to US-based mobile phone numbers, but you can send them from anywhere in the world.
To try out the new feature, click the Google Labs link in your Gmail window. Then, find the feature called 'Text Messaging (SMS) in Chat' click the 'Enable' button, then 'Save Changes' and you're ready to go!