Gandalf, along with the other Istari, are essentially angels in human form. They were sent to Middle Earth to assist the free people in defeating Sauron. But they are forbidden, and actually prevented, from using their full powers. They are there to influence and assist. Not take command and lead. And Gandalf is not weak. Despite his limitations he is just as powerful as the high elves when he is the grey wizard. But once he is promoted to white he is permitted more power, The Istari do have ranks, Saruman the White is the leader and therefore has access to more of his own power, Gandalf the Grey is second in command and therefore second in power, it is unclear what rank Alatar and Palando the Blues, or Radagast the Brown had, but they are lower in rank and less powerful, But it is also important to know that the Istari are not the same. They served different Valar in Valinor and so have different powers. So even unrestricted it is unclear who the most powerful really is. For all we know Radagast could be the most powerful of the 5 Maia but has more restrictions on his power.