Perfect square on EVERY ScreenGui? Is this possible?

You can use a UIAspectRatioConstraint to achieve this. You can insert it in a GuiObject.

Made 100% by @Eternalove_fan32

--Variable
local ScreenGui = script.Parent.ScreenGui
--Create a new Frame
local Frame = Instance.new("Frame",ScreenGui)
--Change the Properties of the new Frame
Frame.AnchorPoint = Vector2.new(0.5,0.5)
Frame.Position = UDim2.new(0.5,0,0.5,0)
Frame.Size = UDim2.new(0.25,0,0.25,0)
local function SetFrameSize () --You can also replace everything in the function is by Frame.SizeConstraint and here that what you prefer

local Choice = math.random(1,2)
if Choice == 1 then
    Frame.SizeConstraint = Enum.SizeConstraint.RelativeXX
elseif Choice == 2 then
    Frame.SizeConstraint = Enum.SizeConstraint.RelativeYY
   end
end
SetFrameSize()

Here you can skip from line 10 to 15 and select only one of the two relatives and replace them like this:

--Variable
local ScreenGui = script.Parent.ScreenGui
--Create a new Frame
local Frame = Instance.new("Frame",ScreenGui)
--Change the Properties of the new Frame
Frame.AnchorPoint = Vector2.new(0.5,0.5)
Frame.Position = UDim2.new(0.5,0,0.5,0)
Frame.Size = UDim2.new(0.25,0,0.25,0)
local function SetFrameSize () 

Frame.SizeConstraint = Enum.SizeConstraint.RelativeXX -- (Here I changed the script)or Frame.SizeConstraint = Enum.SizeConstraint.RelativeYY

   end
end
SetFrameSize()

You have to copy the code and put it in the StarterGui without anything else. I hope this has helped you ^-^ (Your @Eternalove_fan32)