I do think BA (and other airlines) could be slightly cleaer about this. When reading the small print on the link above you get the impression you can only take a guest into FIRST if your Gold and they're on a BA flight:


Gold members can use Executive Club departure lounges worldwide and FIRST++ lounges and may also bring one guest+++ when flying British Airways.

++FIRST lounges are available for use by members travelling on oneworld partner airlines only.
+++ Applicable to departure lounges only.



Where as on the on http://www.oneworldalliance.com/satellite/tier.cfm it states:

As a Gold member, you will also be entitled to the following benefits when flying on oneworld alliance airlines:

Access (with one guest) to more than 230 airline lounges across the oneworld network, including First Class, Business Class & Frequent Flyer lounges regardless of the class you are flying that day.




Reading on I never realised Gold's access to Arrivals Lounge was availible when traveling longhaul BA or Qantas only