Last fall, when creating a Gather Town world, I focused largely on making it aesthetic/pretty. I endorse that as the right choice at the time, for the particular goals I had.
But, a recent update I've made is making extremely liberal use of gather town "conversation spaces" is a pretty valuable. (i.e. you can make an small area count as a "shared conversation node" where only people inside can listen. And you can make those relatively small.
My current guess/hope is that if you make the map mostly such places, they'll feel like the natural place to have conversations, such that it's actually less convenient to have a larger conversation.
I'm not sure that fully solves the problem, where often you have people who don't have a particular conversation they want to have yet, and lurking is actually a reasonable choice. I think the solution of "pay attention to when people are moving around the outskirts of a conversation, and take that as a signal to start a new conversation" is a partial improvement.
(I also think Gather Town is approaching the point where you can use their default editor to make reasonably aesthetic maps that fit particular design choices)
Someone once wrote a LessWrong post saying that people, individually polled, will prefer small group conversations at events – but in practice form large clumps where the majority of people are disengaged but can't coordinate to leave in chunks, probably because coordination competes for a floor contested in proportion to the group's size. (Link welcome, I can't find it.) Gathertown seems to exacerbate this because you can stack people more densely.
I have two suggestions:
(And one tongue in cheek – Gathertown should have escalators or gusts of wind that tend to slowly scatter people, and they need to actively choose to stay with a group or conversation.)
It would be most effective if the host decided on a strategy and broadcast it to participants as the default, but if they don't it seems fine for participants to advertise that they'll be using a particular strategy to avoid this trap.