How do Lutheran discern Law and Gospel in the Lord's Prayer?

In the Lord's Prayer, Jesus famously says: Matthew 6:12 NMB > And forgive us our trespasses, as we forgive those who trespass against us. This seems to be a *law* thing, as it means "unless you forgive, you will not be forgiven". How is it then, that Lutherans (or other Evangelical/Protestant Churches) understand this in a *Gospel* sense, where this becomes a promise telling us "forgive because you too have been forgiven", as Luther says in the Small Catechism .