I love Gleam, but I would start with Elixir if you're interested in learning about how powerful the BEAM & OTP are.
There's not much documentation/resources around OTP in Gleam. When I was playing around with it I often found myself referring to the Elixir docs and then 'translating' that knowledge to Gleam's OTP implementation.
Gleam is still very new so this is totally understandable, and both are great languages so you'll likely have a lot of fun learning either of them.
Erlang is a much better language to learn if you're interested in learning about the BEAM and OTP, and the book "Programming Erlang"[0] is an excellent resource for learning it.
I've used Elixir since 2015 and in fact learned it first. I still think "Programming Erlang" is a much better book than any other for actually learning Erlang and BEAM/OTP principles. Erlang as a language is simpler, leaving more time and energy for learning the actual important bits about OTP.
There's not much documentation/resources around OTP in Gleam. When I was playing around with it I often found myself referring to the Elixir docs and then 'translating' that knowledge to Gleam's OTP implementation.
Gleam is still very new so this is totally understandable, and both are great languages so you'll likely have a lot of fun learning either of them.