Preheat a grill pan over medium-high heat and lightly coat it with cooking spray.
In a small bowl, mix 1 tsp garlic powder, 1 tsp ground cumin, 1/2 tsp salt, and 1/4 tsp black pepper.
Sprinkle the spice mixture evenly over both sides of the 8 chicken thighs.
Place the chicken thighs on the hot grill pan and cook about 10 minutes per side, or until cooked through and internal temperature reaches 165°F (74°C).
While the chicken cooks, prepare the relish: in a medium bowl combine 2 cups cubed pineapple, 1/2 cup corn kernels, 1/3 cup chopped red bell pepper, 1/4 cup thinly sliced basil, 3 Tbsp chopped red onion, 1 Tbsp cider vinegar, 1 tsp sugar, and 1/4 tsp salt. Stir to combine.
When the chicken is done, transfer to a platter and spoon the pineapple-corn relish over the top. Serve immediately.