Stir together ginger, garlic, and two-thirds of scallions in a large bowl.
Heat grapeseed oil in a small saucepan over high until oil is shimmery and hot but not smoking, about 2 minutes. Pour hot oil over scallion mixture. The scallions will sizzle, turn bright green, and wilt almost immediately. Let sit 5 minutes before stirring in remaining scallions. Stir in soy sauce, vinegar, pepper, sesame oil, and sugar; season with salt. Let sit to allow flavors to marry, 15 minutes. Taste and adjust seasonings.
Meanwhile, cook noodles according to package directions. Drain and toss with as much or as little scallion sauce as you'd like in bowl.
Divide noodles among bowls. Top with sesame seeds and serve with chili oil alongside.
