Honestly, this is not a very good question. Can you define GDPR-compliant?
GDPR does not impose many restrictions. GDPR says you have to tell the publishers what you do, and make sure 3rd parties do what you want them to do.
Are you GDPR-compliant if you have an Excel- file with adresses and (emergency) phone numbers of the persons in your group? Or, if you decide to put that into the cloud (whichever cloud)? If you send a copy of the meeting schedule per e-mail or messaging app to someone who has a part?
There are basically 2 ways to use TB
Stand alone with all data on your computer. This restricts possibilities esp with mobile devices.
Through the cloud. TB uses Dropbox. Or you could send the database file around and keep it out of the cloud. The latter does maybe not work well with mobile devices.
In all these ways you have to wonder if this is GDPR compliant.