Whe have: Vitality, Hunger and Energy, as we lose Energy (By pressing shift) we can't run anymore, vitality we loose when we are getting hurt, when starving, or when going to water (as we can't swim) and hunger we start to starve if we don't eat.
I really cant help you with a health gui but I have this it might come in handy
https://www.roblox.com/library/870578477/Health-Bar-GUI?Category=Models&SortType=Relevance&SortAggregation=AllTime&SearchKeyword=health+gui&CreatorId=0&Page=1&Position=1&SearchId=c5966886-6256-4983-8e0d-a30ec99b4683