Yes, gearbox oil in engine smoke and smell. Best you can do, is as soon as possible, completely drain the tank and replace with fresh petrol and correct two stroke oil. However, it may need a period or a few tanks to get rid of all gearbox oil. Also check spark plug for carbon build up. If done promptly and properly, I don´t think your engine has taken any damage. When I was young, I ran hours with gearbox oil in petrol without noticing any permanent damage.