GC vs reference counting has nothing to do with determinism. For the same inputs, the same allocation profiles and same GC events will happen.
Note that CPython's reference counting is by means automatically simpler than OCaml's incremental GC. CPython has heuristics to detect cycles (which normally cause memory leaks in a pure reference counting system), as well as a number of heuristics to optimise performance based on locality and temporal properties of object allocation. All of these contribute to making the CPython runtime less predictable (but faster).
Note that CPython's reference counting is by means automatically simpler than OCaml's incremental GC. CPython has heuristics to detect cycles (which normally cause memory leaks in a pure reference counting system), as well as a number of heuristics to optimise performance based on locality and temporal properties of object allocation. All of these contribute to making the CPython runtime less predictable (but faster).
The OCaml GC is extremely predictable and has very few tricks. It's explained in one chapter of Real World OCaml: https://realworldocaml.org/v1/en/html/understanding-the-garb...