1. Preheat oven to 300ºF. With a serrated knife, very carefully cut the bagels into slices between 1/8-inch and 1/4-inch thick (8 to 10 slices per bagel). Spread in a single layer on baking sheets. Generously coat all over with cooking spray, garlic salt, and pepper. Bake until brown and crisp, 18 to 20 minutes. Let cool on pans.
2. Meanwhile, toss the cucumber with the salt in a colander; let drain 30 minutes. Press and blot dry with paper towels. Mix in a bowl with yogurt, sour cream, oil, vinegar, garlic, dill, sugar, and pepper. Serve with bagel chips for dipping. Dip keeps refrigerated for up to 4 days. Chips keep at room temperature in a paper bag for up to 4 days.
Tip: Lay the bagels flat side down and cut through the side, keeping the knife flat, to slice thinly.