Here’s my proposal for APA format:
Author, Initials (Date of substantial commit used). Title [Computer software]. Pavlovia. URL.
This is based on the information from Reference List: Electronic Sources // Purdue Writing Lab.
I think the best URL is to the Pavlovia page (since it is easy to get to Gitlab from there) or an OSF/ Zenodo DOI.
e.g. Morys-Carter, W. L. (2021, May 18). ScreenScale [Computer software]. Pavlovia. https://doi.org/10.17605/OSF.IO/8FHQK.