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.
Definitely what I would use today.
Jsonnet got rid of "up" and a few other details and suddenly things are much less likely to blow up in your hands.
curiosity: I once wrote a GCL function that computed a PNG data url (the thing that looks like '<img src=data:image/png;base64,iVBORw...' and can be natively opened by your browser) whose content was a rendering of the mandelbrot set. It took several GB of RAM to evaluate. Didn't port it to jsonnet yet...