Rosalina's gravity shield will probably be rather effective with Link/Toon Link's bombs, Peach's turnips and Mega Man's Metal Blades. There is an interesting question, though. Will Rosalina be able to deflect items/ item projectiles like the ones above? For example, let's say that Rosalina is between Mega Man and Samus. Will Rosalina be able to deflect both the Metal Blade and the Charge Shot, or will she pull in the Metal Blade and only deflect the Charge Shot?
I think that the likely reason is that there are certain qualities to the Metal Blade. It's in a normal projectile state, until it hits the ground or a platform. It's then an item. Rosalina can deflect the item if it's in the projectile state, but will pull it in if it's in the item state. Since bombs and turnips are always in the item state, she'll draw them in.