For the longest time I assumed the Falcon 9 legs use pressurized helium to extend.
But gravity by itself would cause them to extend---so long as you first kick them out from their stowed position.
For that, there are little pushers beneath the telescopic leg cylinders.
Drag would be minimal at the low speeds seen when the legs deploy (just just just short of touch down, when the rocket is moving too slow for drag to measurably fight gravity).
So... while it's clear the pushers would need a pressure source to extend---and the helium would come into play there---it's not at all clear that you'd need pneumatics for the large telescopic cylinders...
...Especially when you consider the slow clumsy way in which the legs are often seen to extend on video. Gravity alone could easily produce that.
Can someone confirm if the pressurized helium is for a) both the pushers and the telescopic cylinders or b) just the pushers?
My guess right now is b, but my guesses are as bad as they come, and it'd be nice to confirm this.