I did drop monsters into them. That tooltip informed correctly.
Cullis gates only convert things you drop into them, into essence. It happens at once, when the thing lands in the purple area of the gate. Thing is destroyed, essence generated, and from there it does what essence does.
I just haven’t seen these gates release Essence. I’ve had to keep my cursor nearby to extract Essence.
I actually would prefer that gates don’t release essence, but instead destroy essence that isn’t extracted after some short enough delay, for gameplay balance.