GEL batteries are cyclic just like AGM batteries completely closed and maintenance free. As the name gives away, gel is added to the acid. Thus the electrolyte forms in a gel-like substance that can not leak.
Due to this method of construction, the battery is leak-proof and no gas can form during normal use. The small amount of gas that can be generated is flown back to the electrolyte, just as with AGM batteries. This way, the battery never needs to be refilled and it should not be ventilated.
GEL batteries are more expensive than AGM batteries but they also have a longer life. They can tolerate more loads.