CS gas is a dry powder having the appearance and behaviour of smoke. It will clean off of the equipment relatively easily, but you need to take the same precautions you would if you were knowingly going into a very dusty environment -- that is, it's better to have a sacrificial filter on the lens than to risk scratching the front element, avoid changing lenses (for the sake of rear elements, the sensor, shutter, mirror hinges, etc.). The stuff is irritating to the mucus membranes (and will burn sensitive skin if you're wet or sweaty), but otherwise harmless (other than the potential for abrasion).

A sealed camera shouldn't pose any problems, but consumer/prosumer level stuff would probably benefit from the rain hood, just from the dust exclusion point of view. Oh, and keep whatever camera bag you may be carrying tightly closed, and do give everything a good going over with a vacuum cleaner before putting it away, lest you accidentally transfer any of the evil stuff to a sensitive area weeks later.

Be prepared, though, for pepper spray as well. That's a liquid, and will ingress the same way as any liquid, but its oily nature may be damaging to some rubbers and rubber-like synthetic components (seals, grips, etc.). If you happen to see that stuff coming and don't have a real enclosure for your SLR, you might want to put it away and use a cheap P&S for a few shots.

Oh, on a side note: if someone with a shield and a truncheon tells you to move, move. It's not that they're fascists out to trample your rights (though, undoubtedly, some are of that nature), but that my own experience with riot duty while in the Canadian Forces tells me that at least half of them will be on the verge of soiling their drawers -- they're in a highly adrenal state, expecting rocks, bricks and Molotov cocktails, and reason will not work with them until things calm down a bit. It's not a good time to assert your rights, if you know what I mean.