Could Gandalf have won the war alone?

It is said at a number of points in the story (e.g. by Gandalf himself at the "Last Debate") that there will be no hope for the free people of Middle Earth if Frodo's mission fails. However, I cannot really imagine that Gandalf did not have a "Plan B", and would have surrendered or died without doing anything in case the last battle at the Black Gate failed. By the way, otherwise it would not have been ok in my opinion to send 7000 men to an almost secure death.

My idea is the following: If the battle at Morannon turned out to be desastrous, Gandalf would have shown his full power, marched through the lines of the enemies to Barad Dur and threw Sauron down. If you think that this is complete nonsense, I ask you to consider the following points: At his encounter with Aragorn, Legolas and Gimli in Fangorn, Gandalf says that nobody of these three has a weapon that could harm him, and both these warriors and their weapons were certainly much better than the orcs and men of Mordor. Second, Gandalf was able to completely break Saruman's power at Orthanc, and Saruman - like Sauron - is a Maia from his origin.

So I think that there was indeed some hope for the "West" beyond Frodo's "mission impossible" to Mordor.