There are xsd and schematron schemas [1], see docs generated from the xsd [2]. The GDTF editor on https://gdtf-share.com [3] also has deeper internal validation some of the data logic.

[1] https://github.com/mvrdevelopment/spec

[2] https://gdtf.eu/gdtf/xsd/xsd/

[3] https://gdtf-share.com/