Gases are different, and their effect on the body is also different. But they all cause damage on contact, so they must be removed from the surface of the body as soon as possible. You will hardly understand what kind of gas you used. Therefore, it is better not to take any additional measures.
In general, it is better, of course, not just with water, but with an ammonia solution, with it many chemical warfare agents interact chemically and lose their activity. But if there is no ammonia (ammonia), then it is better to at least wash them off with water. Of course, you need to take off your clothes. And of course, there are special compositions for degassing, which may include alkali, soda, dimethylamine, adsorbents and something else.
At the military department we were told about some cool teacher of weapons of mass destruction, who demonstrated the effectiveness of such mixtures on himself - he dipped his hand in mustard gas, and then immediately transferred it into a degassing solution. Well, during this time the mustard gas had not yet had time to start acting, and nothing happened to his hand.
The substances used in chemical weapons are actually not gases, they are simply called historically. In reality, the only gas was chlorine, which they guessed to use first of all, but then made more convenient and dangerous compounds for use, which most often are liquids under normal conditions.