In a medium saucepan, combine ketchup, gochujang, white vinegar, honey, soy sauce, ginger paste, and garlic paste. Bring to a gentle boil over medium heat, then reduce and simmer for 5 minutes until thickened.

Place chicken thighs in a gallon-size plastic bag, add a few spoonfuls of the BBQ sauce, and season with kosher salt and pepper. Seal the bag and massage to coat the chicken. Marinate for at least 1 hour in the refrigerator.

Preheat the grill to medium-high heat, around 400°F (200°C). Let marinated chicken sit at room temperature for 10 minutes.

Grill the chicken thighs for 4 to 6 minutes on each side, or until the internal temperature reaches 165°F (75°C). Brush with BBQ sauce during the last 2 minutes of cooking.

Remove from grill, let rest for a few minutes, then serve with remaining BBQ sauce.