This is required for CHERI systems such as Arm Morello, where gsize is not large enough to hold a pointer. For all other architectures this should not result in any functional changes since gsize is already the same size (and I believe for most architectures even the same type) as guintptr.
This is fully ABI compatible and should also be fully API compatible
except for some rather unlikely examples such as someone happening to use
decltype(g_atomic_pointer_add(&a, 0)) in a C++ context
where it changes name mangling of some function.