In free-threaded builds, running with PYTHON_GIL=0 or -X gil=0 will now disable the GIL. #116322 and #116329 track follow-up work to re-enable the GIL when loading an incompatible extension, and to disable the GIL by default, respectively.
In order to support re-enabling the GIL at runtime, all GIL-related data structures are initialized as usual, and disabling the GIL simply sets a flag at startup that causes take_gil() and drop_gil() to return early.
With PYTHON_GIL=0 set, I spot-checked a few tests and small programs that don't use threads. They all seem to run fine, and very basic threaded programs work, sometimes. Trying to run the full test suite crashes pretty quickly, in test_asyncio.

Issue: gh-116167

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This post discusses the option to disable the GIL in Python using the PYTHON_GIL environment variable or the -X gil=0 flag. It mentions follow-up issues related to re-enabling the GIL when loading incompatible extensions and disabling the GIL by default. The impact of disabling the GIL on different programs is also mentioned.

gh-116167: Allow disabling the GIL with `PYTHON_GIL=0` or `-X gil=0` by swtaarrs · Pull Request #116338 · python/cpython