Oh yes, those gates will kill you, aliens, anything. The dialog does warn this
Yes, I knew it would happen, but I was having a chain of face-palm moments and this was the last one. Just before that I had finished the whole level and my scout was waiting to pull the last lever when I decided to have my captain kill one last Hunter to get my kill count up, even though the Cap only had about 20 HP left. Of course he missed his second swing and got counter-attacked and killed. Greed, just say no.