Use the $parents array, the grandparent would be $parents[1]. You may also be able to use $root if the grandParent object in your example is the topmost parent.
From the docs:
$parents
This is an array representing all of the parent view models:
$parents[0] is the view model from the parent context (i.e., it’s the
same as $parent)$parents[1] is the view model from the grandparent context
$parents[2] is the view model from the great-grandparent context
… and so on.
$root
This is the main view model object in the root context, i.e., the
topmost parent context. It’s usually the object that was passed to
ko.applyBindings. It is equivalent to $parents[$parents.length – 1].