How to rotate GUI on a point?

I'm looking to rotate a gui with a particular point acting as the center of rotation. Unfortunately I don't know how to go about doing this... Here's an image depicting the situation: http://i.imgur.com/xPkPbzL.jpg

Obligatory code:

function rotateFromPoint(gui, x,y, angle) --x,y being the point on the gui (0,0 being corner)
	--and here I haven't a clue how to offset the rotation point and all that jazz.
end

If you took trigonometry, you would know that in a Unit Circle, the coordinate of any point on that circle is equal to (Cos(Angle),Sin(Angle)). "Angle" being the angle of the line drawn from (0,0) to the coordinate of the point. So in order to rotate a Gui around a point, you would have to code it like this:

function rotateFromPoint(Gui,X,Y,Angle,Radius)
	local XPos = X + math.cos(Angle) * Radius
	local YPos = Y + math.sin(Angle) * Radius
	Gui.Position = UDim2.new(0,XPos,0,YPos)
end

Hope this helped!

Note: XPos and YPos are going to be the offset values, not the scale values.

Link(s): Unit Circle