Gel filters? Sigma 15-30

[excessnoise]

I've never used gel filters. I bought some glass filters 77MM (if my memory is right) that do attach to the hood on the Sigma 15-30 but since they aren't ultra slim, it causes vignetting, so I lose some of the wide angle. Sigma recommends gel filters for the rear (approx. 1" square). What is a good gel filter? Where do you buy them? Is there an equivalent to a polarizer filter in gel? I like the lens but without some type of filter, pics are washed out Maybe there is a good PS pluggin to simulate a polarizer. I tried a demo of Digital Films 55M but couldn't get the results I wanted without a help file.