From what you have typed it looks Legitimate. If the email is from a Google email address and the document is linking to a Study document located in the Google top level domain then I feel it is not spam. If you are concerned, you can send an email or contact Google to find out if it is legitimate. I went ahead and visited the link and it does mention Gift Card and has a link to get started right on Google.
The email seems really strange, I think it's a scam! Most unlikely, Google would not send that kind of email, I think so!