The Gusto integration covers the core handoff well: run payroll in Gusto, and the resulting journal entry, gross wages, taxes, deductions, posts into FreshBooks' books automatically, so you're not manually re-entering payroll totals every pay period.
What doesn't carry over is job-costing detail. If a service business needs to know which project or client a contractor's or employee's hours actually cost against, that mapping has to happen separately, usually by someone cross-referencing time entries against the payroll run by hand.
The other gap is timing visibility. Gusto runs on its own payroll schedule, and the journal entry lands in FreshBooks after the fact, which is fine for bookkeeping but doesn't help a business that wants to see payroll cost against a project budget before committing to it.