"God hasn't forgiven the gunman, why should the Amish?"
Because the Amish are not deities, and because Jesus told them to forgive the gunman.
According to the teachings of Jesus, his followers are called on to forgive and to leave judgment of others to their deity.
The Amish would tell you that they cannot know whether God has forgiven this murderer or not, but that they, themselves, are called upon to do so by the teachings of Jesus. Jesus was very clear about this, and the Amish are very clear in their intent to follow those teachings.
You can find everything you need to know about what the Amish believe in Matthew. It's all there, clear as a bell.
You may not believe exactly what the Amish believe, but they sure as heck follow what they believe...to the letter. No doubt, some in the affected community are struggling with this forgiveness. They're only human, after all. But, the community, as a whole, believes in, and follows, the teachings of Jesus, himself. They're devout in that.
Since you're not Amish, they wouldn't expect you to do the same.