How to make entire model rotate in viewport gui?

With help from the roblox dev site, I got this that can rotate a piece, but not a model. I'm really new with in pairs stuff, but I don't know how to use it yet.

Script in model

local part = script.Parent

local incr = 360/20

-- Rotate the part continually
while true do
	for deg = 0, 360, incr do
		-- Set only the Y axis rotation
		part.Rotation = Vector3.new(0, deg, 0)
		-- A better way to do this would be setting CFrame
		--part.CFrame = CFrame.new(part.Position) * CFrame.Angles(0, math.rad(deg), 0)
		wait()
	end
end

I think welding all the parts of the model to script.Parent using weldconstraints should work.