They are a spring, and they are carried by MSC and small parts and so they are legal (see the restricted parts list).
However, there are still restrictions on thier use. Only so much energy may be stored in them at the beginning of the match (so you may not be able to begin the match with them compressed). See Rule M11. They may also not be used to launch boxes (K10). And of course, S9 applies.
The posts you guys site don’t say that gas shocks are springs. Would they have to be purchased from MSC of SPI then? The person in the post mentions off of a station wagon or SUV, and the reply is sort of vague, just saying they are legal. Assuming they had less than 20 ft-lbs at the start of the match, any shock should be legal, right?