The Dropbox Sign add-on for Gmail allows you to easily sign attachments that are sent to your Gmail.
To sign attachments in Gmail:
Open the email with the attachment or attachments you want to sign.
Click the Dropbox Sign logo in the sidebar to open the add-on.
De-select any attachments you don’t wish to sign, then click Next.
Complete any fields and add your signature, as you would a ‘Just me’ signature request.
Click Sign in the upper-right once it’s completed, and you’ll be redirected to your email in Gmail.
Click Attach document to automatically update the attachment and respond to the email.