Maybe Javascript? A lot of web tools support Javascript config files. There's this nice-looking effort to provide a hermetic execution environment for them: https://github.com/jkcfg/jk and if you use Typescript you get an extremely good static type system too. Plus the language is already very well known with loads of tool support and documentation.
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...
