macOS Catalina command line tools and gcc errors during installation

It is not your command line tools which are failing. Quite obvious, this is a compiler error and it seems like the python library was developed against a different set of headers - the compiler merely states a mismatch.

So gcc is working as expected - the python library‘s C extension is outdated. I fear there is little you can do aside from what you presumably already have done, namely opening a ticket in the MuJoCo-py project.

P.S. It would be nice if you added the information that you already opened a ticket in your question. Even if you did so after you asked here, you can always edit your question.