Why does this gui crash studios?

Simple, the while true do loop is attempting to execute without any delay time. Basically it's trying to run multiple operations at once infinitely, and that causes roblox crashes. Obviously the simplest way to fix it is by putting a wait(Seconds) somewhere in the loop, preferably at the beginning or at the end, like so:

while true do
local HealthNum = game.workspace.Cake.Humanoid.Health
script.Parent.Text = HealthNum.."/100"
wait() --This makes the loop wait 1/30th of a second
end

Hope this helped!

A smarter way to approach this would be to do the following:

Humanoid = game:GetService("Workspace"):WaitForChild("Cake").Humanoid
Humanoid.Changed:connect(function()
script.Parent.Text = Humanoid.Health.."/100"
end)