Follow the steps to get it done:
1. Login to your Google Voice account.
2. Click on the gear icon in the right corner of the screen.
3. Select the Voice Settings.
4. On the settings page check the "Forward the text messages to my email" option.
5. Hit the SAVE button and you are done.
Issue : Some Google voice users have complained that they are receiving text via (GV) Google Voice in their Gmail even without enabling the option. For this you can take a look what Google Employees say about this in Google Voice Forum