You don’t have to choose a target for Gold Rush. However, if you do, and that creature is an illegal target at the time Gold Rush tries to resolve, it won’t resolve and none of its effects will happen. You won’t create a Treasure token.

Count the number of Treasures you control as Gold Rush resolves to determine how big a bonus the target creature receives. That bonus won’t change even if the number of Treasures you control does during the turn.