"The gate is closed."
If you said 「門が閉まるぞ！」,「門が閉まって(い)くぞ！」,「門が閉まって(い)るぞ！」or「門が閉められ(て)るぞ！」seeing a gate closing, I would take it as "Hey, the gate is closing!"
閉まる may mean "closes", "be closing" or "can close." 閉まっている may mean "closed" or "be closing." 閉まった may mean "have been closed" or You find the door able to close.
You can tell them in context. However, unfortunately, I can't explain it in English. I hope someone else could clearly explain it. Anyway, in learning Japanese, especially for beginners, I think it would be better to put aside them.