Best Of

@Simon Cardnell
few ways you can do this

1. in your exisitng flow, get constituent custom fields by category (https://developer.sky.blackbaud.com/api#api=56b76470069a0509c8f1c5b3&operation=ListConstituentCustomFieldsAllConstituents), which will give you the constituent system record id of those that has this custom field; use this array of constituent system record ids to filter your gift list
2. rebuilt your flow using Query API's gift query, that allows you additional filter on constituent custom field.