Although the Gigantiform cast as a spell targets a creature, the Gigantiform put onto the battlefield as a result of the first one's triggered ability does not. It may be attached to a creature with shroud, for example. However, it may not be attached to a creature that couldn't be enchanted by it, such as a creature with protection from green. You don't choose which creature it will be attached to until it enters. Once you choose which creature to attach it to, it's too late for players to respond.