If the creature targeted by the Gigantiform spell is an illegal target by the time Gigantiform resolves, the spell is countered. It won't enter the battlefield, so you won't get to search your library for another Gigantiform.

Gigantiform overwrites all previous effects that set the enchanted creature's power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after Gigantiform becomes attached to it will overwrite Gigantiform.

Effects that modify the enchanted creature's power or toughness, such as the effects of Giant Growth or Glorious Anthem, will apply to it no matter when they started to take effect. The same is true for counters that change the creature's power or toughness (such as +1/+1 counters) and effects that switch its power and toughness.

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 the battlefield. Once you choose which creature to attach it to, it's too late for players to respond.