In a small bowl stir together the maple syrup and whole grain mustard until combined and set aside.
Lay 4 slices of the bread on a clean workspace. Divide the slices of turkey among the slices of bread followed by the slices of ham. Top with the Gruyère.
On the remaining slices of bread, spread 1 tablespoon of the maple syrup mixture on each. Close the sandwiches and gently press down to keep the sandwich together.
Working one at a time, dip each sandwich into the egg mixture then carefully flip it so it’s coated on both sides. Repeat with the remaining sandwiches.
Add the oil to a sauté pan, heat over medium heat until the oil looks shiny. Working one sandwich at a time, add to the oil and fry until golden brown, turning the heat down if the sandwich browns too quickly, about 3 minutes.
Carefully flip and continue to fry until golden brown and the cheese is melted, about an additional 3 to 4 minutes.
Remove to a cooling rack set over a paper-towel lined baking sheet and repeat with the remaining sandwiches.