I'm far from an expert, but I think it is important to remember that it is not the radiation itself that contaminates the water, but physical particles and possibly gases that are radioactive that contaminate the water. Remove them, and you remove the radioactivity.

The Gravidyn is spected to remove particles greater than 0.2 (0.0002mm) microns. Thus, if the radioactive particles are greater than this size, they get trapped. If smaller, they don't. I don't know about gases, but the activated charcoal may trap some also.

So in principle, any filter will catch some particles, but the radioactivity of the particle has nothing to do with it. It is what the filter is designed to catch in terms of physical size of particles and types of gases. I assume almost any filter will help to some extent. When in doubt, use the best filter available.

The real concern to me is that if the filter is catching radioactive particles, then it will become increasingly radioactive itself, and eventually become a potential hazard itself.

I don't think actual water itself (pure H2O) is easily made radioactive--- its what is in it that is the problem.