The grossest way to go green: stop using conventional deodorant and only use the natural kind (better yet, none at all), stop showering as often to save water and do not wash clothes very often also to save water, then switch to riding a bike as sole transportation method (preferably in a hilly area). These work best for the gross factor when used in combination. 

I know you may have meant gross in a different sense, but greenwashing (claiming to be green when you really aren't) is so gross it makes me sick! (figuratively speaking)

Well, one way that is perceived gross, but really isnt, is to urinate in the shower instead of flushing the toilet. Urine is non toxic, sterile, and causes no problems when it goes down the shower drain. A flush saves water, and save money.

The filthiest thing I've ever heard of is composting your own feces.  While this might be good for your garden, I can't imagine anyone wanting to handle their poop more than they have to.  Perhaps sticking with "if it's yellow, let it mellow.." is the better option in this case. 

In response to Aerodog, I agree that while that one technically is not gross, it certainly doesn't sound appealing. To get the point out about this water-saving practice, Brazil put out a PSA a few years ago:

http://current.com/items/90635590_brazilian-psa-teaches-kids-to-pee-in-the-shower.htm

I had a friend whose mother suggested that we use toilet paper and then place it out to dry so it can reused. This gross, but would also probably lead to a lot of bacteria and other things growing, which could lead to infection. I think I will use recycled toilet paper (made from other paper products) to do my part!