I would suggest, NO GOSSIP, about anyone! Even if the gossip is not
about those working there (or about the one not present during
lunch), any sort of negativity toward others becomes a sore that
festers. You always remember that the other held someone in contempt
(and spoke about it in detail); it makes you wonder if they do the
same when about you when you are not present.
Also, there’s a lot of benefit to a regime of formal politeness in a
mixed environment like yours, though it may not be possible to
implement it anymore.
Aim for old-fashioned courtesy and keep all chat work-related.