What does the kicker do?

Indeed, and that becomes significant insofar as you can resist the mast bending forward at the upper attachment point of the gnav using lower shrouds, but it is much harder to find a satisfactory way of stopping it bending back at the gooseneck.  You could use a baby stay from the foredeck to the gooseneck, but your jib won't thank you for it, and probably your original foredeck won't appreciate the loading either.  Thus the mast bend (and so leech tension) is only partially controlled.

For that reason I'd pick a conventional kicker over a gnav every time if the crew wasn't desperately short on space. I have a gnav on the V3k and my crew loves it even if I don't, but on the Laser 3k, I found I could do a lot just by moving the kicker attachment point on the boom forward, closer to the mast, and uprating the purchase to 16:1.  The boom was up to it (or maybe I was just lucky!).

Why a big boat like the Alto, carbon mast and all,  has a gnav I can't imagine.  Extra friction and extra cost that could be better put to a carbon boom that would be made more feasible by use of a conventional kicker.

Have to remember that the fixed points are the mast foot and the tops and bottoms of the shrouds; everything pivots around those.

If you push up on the end of the boom (to simulate wind loading), there is no positive restraint on the gooseneck moving back a little (not necessarily to reverse the direction of bend, but to reduce the forward bend you previously had).  With a conventional kicker, the gooseneck is held by kicker-induced boom thrust against the positive restraint of the lowers adjacent the gooseneck; the only way the sail clew can then move upwards is through stretch in the kicker or bend in the boom.

As a result, for a given kicker setting, the gnav will allow more upward movement of the boom (permitting increased twist) during gusts than a conventional kicker would.