For the marinade you can use a mortar and pestle or a blender or small food processor. If you are using a blender/food processor simply add the garlic, Gochugaru, olive oil, and yogurt and blend. If you are using a mortar and pestle chop the garlic much finer and add the olive oil little by little as you mash the garlic until it is smooth. Then add the rest of the ingredients. The consistency should be a bit thinner than mayonnaise. Coat the chicken generously with the marinade and refrigerate for as long as you can, preferably overnight. Before you cook the chicken season both sides generously with salt and pepper.