The reason it's not in the language tour is because it's not part of the language itself. There's no async specific syntax or feature in the language itself. They all depend on the target since gleam can compile to erlang or javascript. If you compile to erlang you can use gleam_erlang and gleam_otp to leverage OTP. If you compile to javascript you use gleam_javascript and have to work with promises. It's definitely not just an afterthought and gleam_otp recently had a big 1.0 update.