Heat a panini press. Spread 4 of the brioche slices with mustard and top each one with 1 slice of Gruyère; divide all of the smoked salmon and the lemon zest between the 4 slices. Season lightly with salt and pepper and cover with the remaining slices of Gruyère. Close the sandwiches and grill for 2 to 3 minutes, until toasted and the cheese is melted. Cut the sandwiches in half and serve.