Yes, the data is encrypted at rest and in transit.
Just FYI, we see these types of questions typically asked as part of the Information Security review during the sales process. If you need further details you'll need to reach out to your Tealium Account Manager so they can share the details provided to your company from the InfoSec review.
Regarding Google Pub/Sub, Tealium can only speak to data in transit from Tealium and it is encrypted using SSL/TLS.
Regarding data at rest, per https://cloud.google.com/security/encryption-at-rest/
Google Cloud Platform encrypts customer data stored at rest by default, with no additional action required from you.