If Gandalf limits himself to the powers and fighting skills shown in the movies Dumbledore probably has a chance, although Gandalf might also win, depending on circumstances.
If Gandalf uses his full power, which was not seen in the movies, Dumbledore would wisely not even start to duel.
By the way, Dumbledore is one of the very few wizards who can use magic without a wand. And he is probably the only one to ever obtain the Elder Wand in a fair duel, without deception. Gandalf is of course ultimately much more powerful, but please don't underestimate Dumbledore