×
1. The roach must go through the plates in the order: bottom, top, middle.
2. I must set up the trapdoors before dropping the gel, and I can't enter the top part of the room after that.
3. When dropping the gel, an unbroken brain path must exist for the roach to me.
4. The graph { roach starting point -> bottom plate -> top plate -> middle plate -> exit } is not planar, so the initial brain path cannot have the plates in the correct order.
5. The only interaction I can have with the room after dropping the gel is killing the brain in an opportune moment, so that the roach switches to unbrained movement and "flows" to the exit, triggering the last(?) plate.
6. But the two possible unbroken brain paths I see go through the plates in either bottom-middle-top or top-middle-bottom order, and neither seems suitable for the brain killing strategy from the previous step.

×It's not clear if you're already accounting for this, so just in case you aren't: you can hit pressure plates more than once without a problem.

×If you're still having trouble, then I'd guess your problem is "how do I kill the roach after it hits the plates".

In that case, the key thing to realize here is that the brain sends the roach down the shortest path to you.