Preheat your Blackstone griddle to medium-low heat. It is crucial to use a gentler heat so the cheese melts completely before the bread burns.
Lay out your 8 slices of bread. Spread a thin, even layer of mayonnaise (or softened butter) on one side of every slice. This will be the outside of your sandwich.
Place 4 slices of bread onto the griddle, mayo/butter side down.
Immediately build your sandwiches directly on the griddle. Place 2 slices of Cheddar and 2 slices of Gruyère (or Provolone) onto each piece of bread.
Top with the remaining 4 slices of bread, making sure the mayo/butter side is facing up.
Let the sandwiches cook undisturbed for about 4-5 minutes. Check the bottom; it should be a beautiful, even golden brown.
Carefully slide your spatula completely under each sandwich and flip. Press down very gently.
Cook the second side for another 4-5 minutes until golden brown and the cheese is completely melted and oozing from the sides.
Remove from the griddle, let them rest for just a minute so the cheese sets slightly, slice diagonally, and serve immediately!