In a large bowl, combine the shredded rotisserie chicken, chopped celery, halved grapes, and sliced almonds.
In a small bowl, whisk together the mayonnaise, plain Greek yogurt, lemon juice, garlic salt, salt, and black pepper until smooth.
Pour the dressing over the chicken mixture and stir until all ingredients are evenly coated.
Slice each croissant in half horizontally. Place a leaf of green leaf lettuce on the bottom half of each croissant.
Divide the chicken salad evenly among the six croissants, top with the croissant tops, and serve.