Combine water chestnuts, green onions, egg, pineapple, gochujang, parsley, hoisin sauce, Sriracha sauce, sesame oil, black pepper, ginger, and fish sauce in a food processor. Pulse until finely chopped and well combined. Add sausage and pulse filling until just combined. Place 4 gyoza wrappers at a time on a clean, flat surface. Fill a small bowl with water. Add about a 1/2 tablespoon of filling to the center of each wrapper. Moisten 1/2 the circumference of each wrapper using a wet finger. Quickly fold each wrapper in half and pinch 5 to 6 pleats along the edge to seal. Place filled gyoza on a plate and cover with a moist paper towel to prevent drying out. Continue with remaining wrappers and filling. Whisk rice vinegar, soy sauce, green onions, sesame oil, red pepper flakes, garlic, and ginger root together in a bowl. Allow dipping sauce to sit at least 15 minutes before serving. Heat oil in a deep-fryer or large saucepan to 375 degrees F (190 degrees C). Fry gyoza 3 or 4 at a time in the hot oil until lightly golden brown, about 3 minutes per batch. Serve immediately with dipping sauce.