Gnat.start_link

You're seeing just the function start_link, go back to Gnat module for more information.
Link to this function

start_link(connection_settings \\ %{}, opts \\ [])

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Specs

start_link(map(), keyword()) :: GenServer.on_start()

Starts a connection to a nats broker

{:ok, gnat} = Gnat.start_link(%{host: '127.0.0.1', port: 4222})
# if the server requires TLS you can start a connection with:
{:ok, gnat} = Gnat.start_link(%{host: '127.0.0.1', port: 4222, tls: true})
# if the server requires TLS and a client certificate you can start a connection with:
{:ok, gnat} = Gnat.start_link(%{tls: true, ssl_opts: [certfile: "client-cert.pem", keyfile: "client-key.pem"]})

You can also pass arbitrary SSL or TCP options in the tcp_opts and ssl_opts keys. If you pass custom TCP options please include :binary. Gnat uses binary matching to parse messages.

The final opts argument will be passed to the GenServer.start_link call so you can pass things like [name: :gnat_connection].