GSE's safety is probably dose related. I believe Grapefruit-Seed-Extract contains poisonous preservatives , but I'm not sure it causes significant health issues at recommended doses. But you definitely should be very wary of taking high doses, especially for extended periods.
The thing is, Candida is such a difficult organism to control, and Grapefruit-Seed-Extract seems to be very effective for some people when all else has failed, that it has its place as an antifungal. You just need to be aware that it is not a completely safe herb, and it will kill lots of bacteria too (which can sometimes be a good thing if you have bacterial pathogens).