In medium sized mason jar or other container with tight fitting blend combine the following: mayonnaise, ketchup, vinegar, sugar, relish, garlic, salt, pepper. Shake or stir to combine.
Heat skillet or panini pan over medium heat.
Meanwhile, butter bread slices. Placing butter sides facing each other. Spread 1 tablespoon dressing on one slice of bread top with corned beef, Swiss cheese and pickles. Top with 1/2 tablespoon of dressing and another slice of bread. Repeat for second sandwich.
Place sandwich on skillet with butter side touching skillet and second butter side facing up. Heat for about 4 minutes. Flip. Cook until golden brown and cheeses are melted.