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I'm going to go from DKuntz information, and stretch this a bit further.

So, let's start with the facts.

1. The Stargate on Destiny is older than any known stargate, at least, in that part of the universe.
2. The Stargate on Destiny is the only known operating one on a ship, at least in that part of the universe.
3. It's common practice for ships to have higher concentration of oxygen than normal. This enables a few cool tricks, including easier space walking, lower overall weight of gas, etc. See this link.
4. The gas coming from the gate is Carbon Dioxide, per the Stargate Wiki.
5. We know that the gates are much more flimsy than the Milky Way gates. A shot produced a hole in one episode.
6. We know that the stagates deal with lots of energy.
7. While most of the gates aren't of huge importance at any one point in time, Destiny's must be more important, because it's where the Ancients wanted to land.

So, putting all of that together, we can assume the following.

1. Closing a wormhole must involve large amount of energy, of a type that could potentially produce a spark.
2. In order to minimize any affect, it seems likely that CO2 is forced near the gate, to keep any fire from occurring at this sensitive time.
3. This CO2 spray also helps to improve the longevity of the stargate somehow, and isn't required if the stargate isn't going to be used for a long period of time.
4. It could also be that the stargate is full of pure CO2 during the active wormhole, and that it's vented out after the wormhole closes, for the same reasons as above.