Gandlaf would blow the socks off of Voldemort! But in a way, I agree with The Unkown. I bet it would be a close battle though. Just imagine......
Voldemort and Gandlaf are standing in the Quidditch feild, but the hoops have been magically removed. (temporarily) Voldemort is on one end of the field while Gandalf is on the other.
There is a judge's table. The sun beams heavily on the three bored and tired judges. Albus Dumbledore, Alastor Moody and for the heck of it, William Shakespeare. The fighting begins with booms and flashses of light. Spells and curses can be heard. The stands are empty, "Just in case it becomes nasty," as Moody had explained ot the eager Hogwarts students earlier. The battle ends with Gandlaf getting a score of 100% and Voldemort getting 0.5%.
You have imagined. 🤘
I infered that an Istari can be killed if their corporeal raiment was incapacitated. I expect that Voldemort COULD win against Radagast the brown or the two blue wizards, but Gandalf the White? Voldemort would have no chance! A more interesting battle would be Voldemort pitted against Gandalf the Grey or Saruman of Many Colours.