The problem is 99% of people have never read the books and don't understand that the wizards of the LOTR universe are literally immortal demi-gods left to watch and guide the mortal races of middle earth by the actual gods. This is why when Gandalf "dies" in the series he comes back, because the powers that be just brushed him off, gave him a promotion, and sent him back.
The movies do a bad job of portraying this, and actually make Gandalf look like a wimp at times (honestly this is the most glaring deviation from the books).
Also the reason Gandalf doesn't just throw around magic and smite everything is because hes not supposed to. The wizards were sent to guide the races of middle earth and are not supposed to directly interfere, a rule that Gandalf routinely bends anyway which is why he was always so at odds with the other wizards (especially Saruman).
In short, Dumbledore wouldn't have stood a chance, you could give him all three deathly hollows and Gandalf would wipe the floor with him.