Very nice release, Gleam looks like a good contender for "high-level Rust".
Small nitpick:
> One drawback of this sound type system is that converting untyped input from the outside world into data of known types requires some additional code which would not be required in unsound systems.
It isn't really consequence of its sound type system, but its runtime representation - assuming it requires type information to be safely constructed and manipulated, you really need to generate code to do so, but the compiler could instead choose to use more dynamic representation, e.g. compiling to ordinary Erlang maps / JS objects.
Small nitpick:
> One drawback of this sound type system is that converting untyped input from the outside world into data of known types requires some additional code which would not be required in unsound systems.
It isn't really consequence of its sound type system, but its runtime representation - assuming it requires type information to be safely constructed and manipulated, you really need to generate code to do so, but the compiler could instead choose to use more dynamic representation, e.g. compiling to ordinary Erlang maps / JS objects.