While the ability to send money using Google Wallet has been available for quite a while in Gmail on the web, you can now send money from within the mobile app. Once you’ve received the latest update, you’ll be able to send and request money within the email compose screen. This feature is not currently available outside the US or for Gmail for iOS.

  • Open Gmail and tap the compose button in the bottom right corner.
  • Tap the paper clip button in the upper right corner.
  • In the dropdown menu, select Send money or Request money.
  • Enter a dollar amount and hit Continue.
  • If you don’t yet have a payment method set up in your Google Wallet account, you’ll be prompted to add a debit card before proceeding. Tap Add Debit Card, enter all the required information and tap Save.
  • Add a memo for what the payment is for, such as coffee, pizza or rent.
  • Tap Attach Money.

Since this is just an attachment to an email, you can add a recipient, subject and message in the email body either before or after attaching the payment or money request. The recipient doesn’t need to be a Gmail user or have a Google Wallet account to receive funds.
Money sent to a Gmail address will be deposited directly into the recipient’s Google Wallet account. Google says that those without a Gmail account will have the option to have the funds transferred straight to their bank account, free of charge.

After the email has been sent, you and the recipient will both receive an email confirmation from Google Wallet containing the amount, the payment method, the recipient, the memo and the transaction ID.

source.