Preheat the oven to 400°F (200°C) and line a large sheet pan with parchment or lightly oil it.
Pat the drained chickpeas dry with paper towels; in a bowl toss them with 1 Tbsp olive oil, 1 Tbsp lemon juice, 1 tsp garlic powder, 1/4 tsp salt, and 1/4 tsp black pepper until evenly coated.
Spread the chickpeas in an even layer on the prepared sheet pan and roast for 20 to 25 minutes, until crisp to your liking; stir once halfway through if they are crowded.
While chickpeas roast, combine the shrimp with 1 Tbsp olive oil, 2 Tbsp lemon juice, 1 tsp lemon zest, minced garlic, 1 tsp black pepper, 1/2 tsp salt, 1/2 tsp paprika, and red pepper flakes (if using) in a large bowl; toss to coat and let marinate 10 to 15 minutes.
When the chickpeas are done, spread the marinated shrimp over them in a single layer and optionally top with lemon slices; return the pan to the oven and roast 8 to 10 minutes, until shrimp are pink and opaque.
Remove from the oven, squeeze a little extra lemon juice over the dish, sprinkle with chopped parsley, and serve warm.
