What kind of gunk are you talking about??? iF it is just regular dirt and grime, window cleaner will work well. To be on the safe side, remove the glass lids from the tank so as not to get any cleaner into the tank.
If it is the white crusty build-up on the glass, muriatic acid works very well. We recently did this on an old glass top for a tank we Aquascaped at a club meeting. Worked like a charm! I would wear gloves and use it outside. Muriatic Acid has some nasty fumes!