At low concentrations, it is good for inhibiting growth of microbes in the water. At higher doses (see their website for details), it helps to "clean up" the digestive tract of pathogens.
Not only have I used it for that purpose on snakes, but also on my husband! He came back from Madagascar one winter with a persistent "stomach bug" that caused chronic diarrhea, nausea, etc. It went on for months and nothing specific could be found. So I started him on as much of the extract in all of his liquid intake as he could stand. The symptoms abated after a week or so. When he stopped taking the extract, the symptoms came back. But the problem eventually resolved between the extract and his own defense system. So I do have some respect for this supplement. I have considered using it as a first treatment on regurgers to knock out everything possible, then use heavy doses of Nutri Bac to put the good stuff back! Makes sense, but I haven't tried it that way yet.
The only negative (besides the taste) is that one doctor told me he thought heavy doses of the extract may change the pH of the snake enough that it should be mentioned to a vet if you seek treatment. That is because an altered pH could require a different dosage of medication.
As far as I know, grapefuit seed extract has NO effect on chemicals (such as chlorine), metals, toxins, etc. I use it only as a teatment against harmful microbes.