Place drained chickpeas in a medium mixing bowl and mash with a fork or potato masher until mostly broken down but still slightly chunky.
Add vegan buffalo sauce, vegan mayonnaise, diced celery, minced red onion, lemon juice, and garlic powder to the bowl.
Stir the mixture thoroughly until all ingredients are evenly incorporated.
Season with salt and black pepper to taste.
Toast bread slices if desired.
Divide the chickpea mixture evenly between two slices of bread.
Top with arugula or lettuce and the remaining slices of bread.
Slice diagonally and serve immediately.