Elixir's type system is still baking(can't define customs types yet), while Gleam's is ready here. Gleam has (familiar to many) battle-tested Hindley-Milner with great type inference, while Elixir's is a different (still sound) set-theoretic one.

But most importantly, Elixir is, and always will be, a gradually typed language. There will always be parts of it that will remain or encourage dynamic typing and that opens the door to ambiguity. In a statically typed language the compiler enforces type annotations and that is where Gleam shines.