Need help Completing My System? NEW

The solution is to check if the health is going down, here is how.

local PreviousHealth = 101
local hum = script.Parent.Humanoid
local RG = game.Players.LocalPlayer.PlayerGui.DamageGui

hum.HealthChanged:Connect(function() -- when changed..
    if hum.Health < PreviousHealth then -- is the health less than the previous health?
      RG.Enabled = true -- open ui
      wait(2)
      RG.Enabled = false -- close ui
    end
    PreviousHealth = hum.Health -- set the health in case it goes up
end)

If this helps, please click accept, if not message me in the comments. Good luck!

You can do this by keeping a variable which holds the previous health, then compare each time when it gets called and check if the current health is lower than the previous health

Example

local hum = script.Parent.Humanoid
local RG = game.Players.LocalPlayer.PlayerGui.DamageGui
local prevHealth = hum.Health

hum.HealthChanged:Connect(function(health)
    if prevHealth > health then -- check if prev health is lower than current health
        RG.Enabled = true
            wait(2)
        RG.Enabled = false
    end
    prevHealth = health -- set the next previous health
end)