Gandalf himself answers this (albeit obliquely) in the Council of Elrond:
[I]t is not our part here to take thought only for a season, or for a few lives of Men, or for a passing age of the world. We should seek a final end of this menace
Fellowship of the Ring Book II Chapter 2: "The Council of Elrond"
Gandalf could have broken the Rules, but that would only have been a temporary solution that would have caused problems down the road; it's worth remembering why the Valar insisted on these restrictions for the Istari:
[T]his the Valar did, desiring to amend the errors of old, especially that they had attempted to guard and seclude the Eldar by their own might and glory fully revealed
Unfinished Tales Part 4 Chapter 2: "The Istari"
Sure, he may have won the battle for the Free Peoples, but he would have postponed the end of the War.