One of the effects of the Swarmkeeper ranger's Gathered Swarm feature is "immediately after you hit a creature with an attack the attack's target takes 1d6 piercing damage from the swarm." What happens on a critical hit? Does this count as additional damage to the attack (i.e. the damage is doubled to 2d6) or is it a separate effect which occurs after the attack and would not be modified? 

It says "immediately after".  You attack, you roll to hit, you get a critical, and your weapon damage is increased.  Right after that, the swarm hits.  It doesn't roll to hit, so it cannot crit.

Quote from Geann >>

It says "immediately after".  You attack, you roll to hit, you get a critical, and your weapon damage is increased.  Right after that, the swarm hits.  It doesn't roll to hit, so it cannot crit.

 That seemed to be the implication to me too, but wanted to check if there was any other thing that I was missing. Thanks 

Just make sure if you're treating it as a separate instance of damage that it forces an extra concentration check as well. In my games we just consider it part of the attack like all the other similar extra damage things from other ranger subclasses, and thus can double on a crit. But I can see that the RAW may indicate otherwise.