If its a one on one physical fight, then Smaug would win 90% of the time, and probably draw much of the remaining 10%. While Gandalf is powerful and skilled in fighting, his true strength is in his wisdom and advice not in open combat.
Gandalf could not do much against a wolf and goblin raiding party, what chance would he have against a beast which destroyed two kingdoms when it was young and tender.
Gandalf does not know about Smaugs missing scale, and his sword is unlikely to penetrate the dragons defenses, and when it does it is unlikely to harm it to any great degree. I also suspect Smaug would be 'powerful' enough to overcome any spell which Gandalf might try to place over him, think back to Glaurung, and how he ravaged the armies of Elf lords of the first age, many of which could stand a chance against maiar (Balrog, Saruman) in open battle.