The ability to create a signature has been added to Google's Inbox.
To create a signature login to
Google Inbox, click the 3 lines to expand your navigation (if needed), then scroll down past your labels to the bottom of the column:
At the bottom of the navigation column, click Settings:
On the settings page, click Signature, move the slider button to On, and then type in your signature:
When you are done typing your signature click the Done link on the bottom right of the window.
Your new signature will be automatically synced across all of your devices.