while wait(0.1) do local Gui = script.Parent Gui.Frame.TextLabel.Font.Random() end
You have to set the property you want to change equal to something. There are 45 values for the font enumeration so you want to get a random number between 1-45.
Gui.Frame.TextLabel.Font = math.random(1,45)
Let me know if this works for you.
You could try:
while wait (0.1) do local Gui = script.Parent Gui.Frame.TextLabel.Font = math.random(1,10) --In the brackets, write your smallest and largest random number end
Try this - it could work! Let me know if it is useful!