Whisk the ketchup, sugar, garlic powder, ginger powder, white vinegar, and pineapple juice together in a small saucepan until the sugar starts to dissolve.
Set the pan over medium heat and bring to a gentle simmer, stirring occasionally so nothing sticks to the bottom.
In a separate small bowl, whisk the cornstarch and cold water until completely smooth with no lumps.
Slowly drizzle the slurry into the simmering sauce while whisking constantly. Cook, stirring, until the sauce turns glossy and lightly coats the back of a spoon but still pours easily, about 1 to 2 minutes. If it tightens too much, whisk in a splash of water or pineapple juice.
Remove from the heat and taste. If it is too sweet, whisk in a little more vinegar. The sauce thickens slightly more as it cools but should stay pourable.
Notes
The sauce is ready when it turns glossy and coats the back of a spoon. Go by the look more than the time.
Store cooled sauce in an airtight container in the fridge and loosen with a splash of water when reheating.