I use it for both. I built a SaaS (ish) that runs online systems for my clients (goods that are shipped, delivered, picked up, digital, subscription or not, etc), so they all have their own Gravity API keys that I manage. They're not all run as "child accounts" under my account; they have to go through their own underwriting, which is a good thing. If they ever stop working with me, they need to be able to take their payment gateway with them too. I run that same system to operate my own business and all my billing (cards and ACH).
I find the integration to be pretty straightforward. I've unfortunately done a dozen different payment integrations, and Gravity was the easiest, and the support most helpful. To this day, my favorite support call was this obscure API question I had, and the lady who answered the phone knew it off the top of her head right away. I was floored. That was in December 2017.
Gravity can work with their own API (Emergepay), or you can use them to connect to Auth.net or USAePay or NMI. The payment world is just a long chain of middleware passing the transaction off to a bigger fish until you hit Visa/Mastercard.
Another client has their own custom system for school management software. Stripe was such a nightmare last fall as the school year kicked off that we've spent the last year integrating Gravity as a replacement. We just onboarded our first new school to Gravity this week as we flesh out the integration and prepare to migrate the rest of the schools over the coming months. It's a hectic time for schools right now, but the savings in transaction fees are enough that they're lining up to switch! $6M+/year wasn't enough for Stripe to answer the phone.
The rates you get with Gravity will vary depending on your risk profile. I've never seen anything higher than the usual 2.9%+$0.30, and it could be as low as 2.65%+$0.10, and probably less if you are 501(c)(3). In-person (card-present) rates are less, and then debit is less, and then ACH is the least. I think my ACH rate is $0.40. But the rates will fluctuate with your type of business.
I don't think Gravity is doing anything magical: they're simply doing the risk-assessment up front, whereas Stripe is doing it after it's too late (from the merchant's perspective, i.e. you cross some arbitrary threshold and oops, your funds are locked and account is blocked. Now beg the bots to let you back in). It lowers the barrier to entry for Stripe, sure, but it also lowers it too much, I think. Do you really want anyone to be able to accept credit card transactions within minutes? It sounds nice....but it's almost too easy, and too easily abused. The prevalence of credit card fraud combined with the ease of getting an access to an infinitely scalable worldwide transaction network is a scary combination. The cost of convenience, I guess.
In my opinion, I'd rather have a 2-3 day turn-around for a payment gateway if that also means I get 1) proper underwriting with a conscious human (oh, the trajedy!), 2) a relatively solid guarantee it will keep working until I tell it to stop, and 3) someone will pick up the phone when I call (or better yet, call me first to check in if something seems off). An added bonus is that's usually the same person, that I know and have seen in real life, and they know me and what I do, and we have context and consistency. Saves a lot of hassle and back and forth.
Does Gravity have all the fancy graphs and reporting and Radar (not that that even works)? Probably not. In my experience, as these systems get too large to ignore, they raise their prices to pay for features you don't want and remove features you do want.
You can do online invoicing and recurring in Gravity, but I don't personally. My SaaS system handles my invoicing and subscription frequencies, as I'd rather my business logic be owned and housed in my database, and not be tied to the payment gateway. I prefer those tradeoffs, but not everyone does.
I know a lot of people want to delegate as much responsibility off to [insert large tech company here], but I've trended in the opposite direction over the years. It's too volatile trying to keep up with all the varying updates, API versions, downtimes, deprecations, buyouts, mergers, upgrades, maintenance, etc. Maybe it costs me a late night here or a busy weekend there, but at least my stack moves at my pace. And my stack is one that I (mostly) understand and control, rather than just hook into an API over here and API over there.
The trajectory in tech for many years was to have as little responsibility in the stack as possible, but charge like you do. And I think that has been slowly backfiring for years, but is really coming to rear its ugly head in the last couple years.
All that to say: I'm willing to pay more for a service that works in perpetuity, and when it doesn't, someone will help. Granted, I've had precious few issues and no outages with Gravity across many years and many, many clients. And now that I think about it, I don't actually know what my credit card rate is for my business. Whatever it is, it's worth not having to be on the front page of HN once a week, scrambling for a solution.
Sorry for the length. Got carried away.