Beware, Gravity screwed over numerous businesses I work with. They purposefully misclassified businesses SICs (a code that defines what very specific category your business is, eg: a bar or a restaurant) to reduce credit card processing fees (interchange fees) for their customers, but were caught doing this for a massive number of businesses.
The card networks and issuing banks that had this money stolen from them went after the businesses that dodged these fees illicitly, with bills of tens of thousands or hundreds of thousands of dollars for fees that should have been paid.
Having previously implemented a variety of other payment providers, I am now looking at a Gravity Payment integration.
Not done yet, but so far pleasantly surprised by how they communicate with integrators.