While having a quick look through old news:

From June 2021 (v0.16) (https://gleam.run/news/v0.16-gleam-compiles-to-javascript/#how-does-it-work)

Much like the Erlang compiler backend this new JavaScript backend outputs human readable and pretty printed source code. It is now included with the compiler and does not require any extra components to be installed to use it.
Rather than attempting to replicate a subset of Erlang’s actor model Gleam uses the standard promise based concurrency model when targeting JavaScript. While this may be disappointing for some, it means that there is no additional runtime code added. This keeps bundle size small and makes it so code written in Gleam can be called like normal from languages such as JavaScript and TypeScript.

Jan 2024 v0.34 (https://gleam.run/news/v0.34-multi-target-projects/#multi-target-projects) mentions some additional work done to enable multi target projects such as Lustre