You can use both because you use them in totally different ways. Irish moss is a kettle fining- it goes into the boil 10 or so minutes before flameout. Gelatin is used in secondary (or right in the keg if you keg)- you need to mix it fully with fermented beer and then cold crash it for 48+ hours (72 is my standard gelatin time).
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
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