> CUE [...] makes an effort to incorporate lessons learned from 15 years of GCL usage.
The main lesson learned by many people at Google, including the operators of many mission-critical services, was that GCL was far more trouble than it was worth. They used an alternative more mainstream configuration language instead.
which mainstream configuration language?
Just a guess, https://github.com/google/starlark-go perhaps? It is a subset of Python.