z-Tree unleashed is permissively licensed under a MPL 2.0 style license. The only additional requirement is that you cite the following paper if you use z-Tree unleashed in an academic context:

Duch, M. L., Grossmann, M. R. P. and Lauer, T. (2020). “z-Tree unleashed: A novel client-integrating architecture for conducting z-Tree experiments over the Internet”. Working Paper Series in Economics No. 99, University of Cologne.

The full license is available here.