I am also a BA Gold (Emerald) and I've never been able to select seats for free on the LATAM site. I've resorted to calling the call centre who are happy to oblige.
My experience on the phone with companions (on the same ticket) with no status is that they will be asked to be pay for their seat. Similarly to you find it confusing given Black membership benefits and what typically other Oneworld airlines would do (at least BA, AA or Iberia) in this situation, which is to extend the benefit to the full travelling party.