Place all of the bread slices on a flat work surface. Using a butter knife or small rubber spatula, spread 1 side of each slice with some of the softened butter. Use 1/2 of the butter for this and you'll need the rest later.
Turn over 4 slices of the bread and spread the unbuttered sides of 2 of the bread slices with the mustard, and the other 2 slices with the honey.
For each sandwich, fold 1 ham slice on top of the mustard spread and top with 1/4 of the sliced Gruyere cheese. Cover with the remaining 4 bread slices, making sure that the buttered sides face out. You will have 4 sandwiches.
Heat a large skillet over medium-high heat and melt 1 tablespoon of the butter in the skillet. Transfer the sandwiches to the skillet and cook until golden brown on the bottom, about 2 minutes. Turn and add the remaining tablespoon of butter to the skillet. Cook the sandwiches until the cheese has melted and the second side is golden brown, about 1 1/2 to 2 minutes. Remove the sandwiches from the pan and serve immediately.