They do not happen.
The first time Gelderfang would be encountered, summon and build the location Rapture of Rupture, then shuffle Gelderfang into that location.
Before you act, each character at your location must succeed at a Combat 25 check or be dealt 1d4+1 Combat damage.
If undefeated, recharge all spells that have the Attack trait and weapons in your hand.
Because it say "would" be encountered, the thing you do replaces actually encountering him. And because he isn't actually encountered, you don't have the encounter steps (like before you act) and you don't resolve the encounter so he isn't undefeated.