Soap. Use a glycerin soap and rub it dry on the GG then buff it off. You can moisten it a little, but not wet - buff off when dry. It doesn't last forever of course, but it works and it's cheap.
Don't use it on your lenses of course.
Pentax used to make an anti-fog lens cleaner but I haven't seen it in a while. It also left a visible residue on the glass 9as it must to work), but it would clean off with regular lens cleaner or alcohol.