Kinda. There's a fair amount similar but things like message passing are implemented as a library in gleam [0] rather than being a part of the core language, so you might say the big things that make erlang _erlang_ are not part of Gleam directly.
> is the JS target as well supported?
The experience is pretty good, yeah! The compiler can emit typescript declarations, there's a vite plugin for gleam, I maintain a frontend framework written in gleam and it's all grand.
> Is performance just the same as Erlang (when compiled to Beam) or there's differences?
That's right, perf is the same.
> Why would someone use Gleam instead of Elixir (they now have optional types - not sure how well those work though)? Is there really demand for more than one modern language on top of Erlang's VM?
Few reasons. Elixir's type system work is very much Work In Progress and could be a long time before we get something "finished". Also the semantics and approach have quite different designs: I'd guess folks that like the gradual type experience Elixir is pushing would not like Gleam's type system and vice versa.
There's certainly room for us both on the BEAM (and we do interoperate too!)