Similarly to bpo-39573 (PyObject) and bpo-40170 (PyTypeObject), I propose to make the PyGC_Head structure opaque in the C API.

See https://bugs.python.org/issue39573#msg361513 for the rationale. In short, my plan is to hide all implementation details from the C API.

The PyGC_Head structure caused ABI issues recently: bpo-39599 "ABI breakage between Python 3.7.4 and 3.7.5: change in PyGC_Head structure". Making the structure opaque would reduce the risk of such ABI issue. In fact, the reporter of bpo-39599 really require to access PyGC_Head structure to write a profiler, so this issue doesn't fix all use cases, but it should benefit to most people ;-) PyGC_Head structure will remain accessible via the internal C API which doesn't provide any backward compatibility warranty.