Almost every quest in Repugia is related to the campaign, so missing this should be hard.
There are only a handful of optional quests:
* Find the kids that are hiding.
* Trade for Creepy Treats cards.
* Bob for eyeballs.
The kids are all wearing bright orange candy corn costumes and are all outside.
The Creepy Treats quest is like any of the others, you will eventually get a duplicate of the card they want and you can trade for one you don't have.
Bobbing for eyeballs is the same as bobbing for apples. Just grab the ones of high value and ignore the rest unless you have time.
By the time you get to the gate you should have these taken care of. When you defeat Araxia, this will pop along with Birdbrain Beatdown in Costume Quest