NABU stands for “network access and brand usage.” However NABU is a kind of catchall category for a range of card-brand and card-association fees.
As with interchange fees, NABU fees are not charged by Gravity Payments or by any credit card processing company, but by the banks and card brands themselves. The credit card processor only collects the fees and passes them through to the card brand and bank.
See our page on card brand fees to learn more.