I disagree. Works are things that we do. After we are saved the good that we do is not our works but Christ in us.Ephesians 2:10. It is a work, but solely after being saved. Since in Christ forgiveness is completely do to grace, then the requirement to forgive in order to be forgiven can only be under the Law. Therefore as I argued it is Law.
To complicate things.....men are not forgiven because they forgive other people (under the Law). There is no provision for forgiveness under the Law.
Christ did not come preaching the Law but proclaiming that God's kingdom has come. If we do not forgive others we will not be forgiven. This does not mean that in order to be forgiven under the Law we must forgive other people.