I assume by 'concurrent GC' you mean multicore parallelism in the same process, in which case, yes, OCaml has it now: https://discuss.ocaml.org/t/ocaml-5-0-0-is-out/10974

And Jane Street have long since moved forward into even more advanced territory. They are now trying to add Rust-like ownership guarantees: https://blog.janestreet.com/oxidizing-ocaml-ownership/