Gross MRR churn is the MRR lost to cancellations and downgrades as a share of starting MRR; net MRR churn subtracts expansion and reactivation from that loss.
Gross = (churn + contraction) ÷ MRR at start · Net = (churn + contraction − expansion − reactivation) ÷ MRR at start
Gross churn shows the leak. Net churn shows whether upgrades make up for it; a negative net churn means the existing customers grew by themselves.
New business is in neither: both rates are about customers you already had.
sumwerk shows both for every month, in the months table, the API and the CSV export.
A restricted, read-only key is all it takes. The first import runs in a few minutes; every number you see opens into the customers and Stripe references behind it.