I am using Zacam because she is creature removal, life gain, burn damage, and artifact and enchantment removal on a stick. By using Zacam as my commander I don't have to build a lot of those things into my deck and can be more stompy than others. Gishath as a commander would feel super risky and I am not sure I would ever be able to get through with Gishath to utilize his abilities. this is very dependent on play groups.