Bran was expecting some kind of spell, since she must have had some reason to believe she was a witch. The only problem was there were so many versions and representations of witches that he had no idea which one she might be. They seemed to get less and less sinister as time passed - witches in the eighteenth century didn't quite measure up to the Wicked Witch of the West, after all - but somehow he couldn't see Gabby as some kind of evil, cackling spell-caster.
He didn't quite understand why she wanted him to cut the apple, but he shrugged and picked up the knife regardless. He raised his hand over the apple, watching with raised eyebrows as a ball of light appeared out of nowhere between her hands. Although he knew people had abilities, this was something completely new. It was hard to keep from staring at the light-thing hovering in the middle of the kitchen, but he regained his wits just in time to realize he was supposed to be cutting the fruit. He brought the knife down directly through the center, but instead of slicing through the blade ended up on the right side of the apple, leaving it unmarked.
He tried again, with the same result. "Wow." So she could cast... what, shield spells? That was a useful ability. "That's impressive, not to mention useful."