Does Gusto sell them your salary data as well?
This is a REALLY good question and something that's hard to appreciate until you actually to build a payroll system. I think a common misnomer is that if you're not doing ML/AI/AR/blockchain/[insert latest technology here], you're not doing R&D.
The domain of Payroll turns out to be an incredible complex business domain. I think Ron Jeffries says it best in his post: http://wiki.c2.com/?WhyIsPayrollHard
The software design of such a complex business domain at scale turns out to be an incredibly hard engineering challenge, and something that is often overlooked when we think about big engineering challenges.
A little known fact is XP and Agile were developed by Kent Beck while working on a Payroll system for Chrysler (In fact, Kent now works at Gusto to help us with our payroll system).