The lounge staff at GVA are often trying to manage capacity, given it's not a huge space and there are a lot of BA flights scheduled. I have repeatedly heard assertions made to others that Silver are not allowed a guest (which is not BA or One World policy) so it would not surprise me if they bent the rules in a similar way for otherwise eligible passengers on non-BA flights.

It's not a huge deal and I get why they try this (and I have never had any issues myself, with Gold) but I've seen this before at GVA and I think it's their way of trying to stop the lounge from becoming overwhelmed. GVA is a status heavy route and at certain times of the day there can be 2/3 flights worth of eligible passengers (plus guests) attempting entry.