A nice little spinner for Garry's Mod GUI.

AngleSpinner.lua

local function mouseToAngle(panel, w, h)
	local x, y = gui.MousePos()

	local globalX, globalY = panel:LocalToScreen(w / 2, h / 2)
	local aX = x - globalX
	local aY = y - globalY

	local cosX = aX / math.sqrt(math.pow(aX, 2) + math.pow(aY, 2))
	local ang = ((math.atan2(aY, aX) * 180 / math.pi) * -1) -- math.deg(math.acos(cosX))

	ang = (ang - 90)%360
	return math.floor( ang )
end

local function CreateSpinner(text, bind)
	local convar = GetConVar(bind)
	local col = SKIN.Colours.Label.Dark or Color(0, 0, 0, 255)
	local img = Material('widgets/disc.png')
	local lastRotation = 0

	local p = vgui.Create("DButton")
	p:SetText(text)

	p.rotation = 0
	
	function p:Paint(w, h)
		local ow, oh = 0, 0
		local angle = p.rotation .. "°"

		if p.down then
			lastRotation = p.rotation
			p.rotation = mouseToAngle(self, w, h)
			if lastRotation ~= p.rotation then
				self:UpdateConVar()
			end
		end

		surface.SetMaterial(img)
		surface.SetDrawColor(col.r, col.g, col.b)
		surface.DrawTexturedRectRotated(w/2, h/2, h, h, self.rotation)

		surface.SetFont( "DermaDefault" )
		surface.SetTextColor( col.r, col.g, col.b, col.a )

		ow, oh = surface.GetTextSize( angle )
		surface.SetTextPos( w/2 - ow/2, h/2 - 20 )
		surface.DrawText( angle )

		return false
	end

	function p:UpdateConVar()
		convar:SetInt(p.rotation)
	end

	function p:OnMousePressed()
		p.down = true
	end

	function p:OnMouseWheeled(sd)
		local val = (p.rotation + sd)
		p.rotation = val%360
		return true
	end

	function p:OnMouseReleased()
		p.down = false
	end

	p:SetSize(100, 100)
	return p
end

Special thanks to DBotThePone for helping me with the math because my brain stopped working at 5 AM!