Starlark has taken over the build system (as it was originally built for) and a small number of other systems at Google. There are a couple of other internal languages that found success in specific domains. But GCL remains the most common config language at Google unfortunately.
I hear the GCL lambdas make for a bad day. Marcel, who created CUE, helped develop Borg, borgcfg, GCL. He talks about the "unfortunate" sentiment in the last video on this page https://cuetorials.com/videos
Funny detail: I really did not understand GCL until I read about CUE and also JSonnet, which I believe are both by post-GCL ex/Googlers. Once I understood those languages, I started to get a handle on GCL. "GCL is like a fucked up JSonnet" is a better angle than "JSonnet is like GCL if it wasn't torture"
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...
