Have you seen it?
I'd turn the translation around.
The Chinese is focusing on the hand (literally: dang1xin1 当心 ya4 轧 shou3 手 - "watch out" "crush" "hand"), I'd focus on the gate. Suggestion: "Watch out, moving gate".
Many thanks to Bernd, who also runs the wonderfully quirky www.sleepingchinese.com.