Preheat oven to 425° Fahrenheit. Reposition oven racks if needed so that sheet pan sits in middle of oven. Lightly grease large sheet pan with spray oil or butter.
Pat chicken thighs completely dry with paper towels, then place chicken on prepared baking sheet. Generously season tops of chicken thighs with salt and black pepper, then flip chicken thighs over and season other side. Rearrange chicken thighs as needed to prevent overlapping or overcrowding, then set baking sheet aside.
Add gochujang, olive oil, garlic, ginger, soy sauce, and sesame oil to medium mixing bowl. Stir well until ingredients are fully combined.
Place rinsed and dried potatoes in large mixing bowl. Add ¼ cup of prepared gochujang sauce to bowl with potatoes, then toss potatoes until fully coated in sauce.
Transfer potatoes to baking sheet with seasoned chicken thighs. Arrange potatoes around chicken thighs so chicken isn't covered, to ensure even cooking. Brush tops of chicken thighs with remaining prepared gochujang sauce.
Place baking sheet in preheated oven. Oven-roast chicken and potatoes 35 minutes, then begin checking doneness by inserting internal meat thermometer into thickest areas of chicken thighs. Continue roasting chicken as needed until internal temperature reaches 160° Fahrenheit.
When chicken reaches desired internal temperature, carefully remove baking sheet from oven and set aside. Allow chicken to rest 5 minutes, then divide chicken and potatoes into preferred servings, top with sliced scallions, and serve immediately.