I am not 100% sure as I read different versions of the GEO method; but I think if you are having a problem and want to try it, the version I read said to empty your tank and make sure it is closed. Then mix 2 cups of water (not fabric) softener in 1 gallon of hot water with 1 cup of laundry detergent. Then use as you normally would and then when full to empty as usual. You can also put some of this in your gray tank if it has an odor also. Anyway the main ingredients are: Water softener; water: laundry detergent. Some people also put a little bleach in it also.
We use Thetford powdered chemical and lots of water when we use the bathroom. Might have to empty it a little more often but have never had a problem with odor or built up "stuff."