(I know Erlang well, but haven't used Gleam)

The library the parent links to says this:

> Not all Erlang/OTP functionality is included in this library. Some is not possible to represent in a type safe way, so it is not included.

Does this mean in practice that you can use all parts of OTP, but you might lose type checking for the parts the library doesn't cover?

No, it means that one specific package only offers bindings to certain parts. It’s the documentation for one library, not the language.