Quote from the Google Wallet Page at http://www.google.com/wallet/send-money/ "Google Wallet is now integrated into Gmail. This means you can securely send money to friends and family in the U.S. - without leaving your Gmail inbox."
This feature will be rolling out to Gmail users ages 18+ in the U.S. over the next few months. Look for the $ icon in your attachment options. You can also get earlier access if your friends have the feature and send money to you.
It’s free to send money from your Google Wallet Balance or directly from your bank account. You can also pay with your credit and debit cards for a low fee of 2.9% per transaction (minimum $0.30). Receiving money is always free.