(note, '+' Shape is the name on the Wiki)
I have had the gates open on me when there are 2 giant lichens left during the last wave. Each one was trapped in a different quadrant by the crystal blocks so I was free to go on my way without having to fight them if I didn't want to.
... the steam overlay likes to duplicate my posts.
The metaphorical internet gate opens early and lets my posts out even though they haven't combined into one giany mega-post yet.
So it's kind of like the lichens when they haven't combined, but the gate opened and they posted even though they're still not ready. Or something.
When the smaller lichens combine to make a big one the game counts them as "killed."
So when everything that originally spawned in the level has "died" and is no longer present in the arena, even if it's because it only vanished in the MANLY COMBINING of a giant super monster, it's all still dead, and the gate opens.
Can't imagine why you'd cut and run when all that's left is a few huge lichens, though. They're hardly hard to deal with with nothing else around...