3D Objects inside Guis?

I was originally not going to answer this question, but I feel that enough people are confused about the topic that I must. ROBLOX's Games team used the a function in the following code to convert the object's position into a vector2. They then cloned an ImageLabel to that position. There was never a 3D object in the GUI. Here is the code, please note that it was made by the Games team, not me.

local function PointToScreenSpace(at)
    local point = Workspace.CurrentCamera.CoordinateFrame:pointToObjectSpace(at)
    local aspectRatio = ScreenGui.AbsoluteSize.X / ScreenGui.AbsoluteSize.Y
    local hfactor = math.tan(math.rad(Camera.FieldOfView)/2)
    local wfactor = aspectRatio*hfactor
    --
    local x = (point.x/point.z) / -wfactor
    local y = (point.y/point.z) / hfactor
    --
    return Vector2.new(ScreenGui.AbsoluteSize.X*(0.5 + 0.5*x), ScreenGui.AbsoluteSize.Y*(0.5 + 0.5*y))
end


PlayerGotCoin_Event.Changed:connect(function()
    if PlayerGotCoin_Player.Value == Player then
        local at = PlayerGotCoin_Location.Value
        --
        local pos = PointToScreenSpace(at)
        local pointSet = {
        PointToScreenSpace(at + Vector3.new( 0.75, 0, 0.75));
        PointToScreenSpace(at + Vector3.new(-0.75, 0, 0.75));
        PointToScreenSpace(at + Vector3.new(-0.75, 0, -0.75));
        PointToScreenSpace(at + Vector3.new( 0.75, 0, -0.75));
        }
        local minx, maxx, miny, maxy = math.huge, -math.huge, math.huge, -math.huge
        for _, pt in pairs(pointSet) do
            if pt.x > maxx then maxx = pt.x end
            if pt.x < minx then minx = pt.x end
            if pt.y > maxy then maxy = pt.y end
            if pt.y < miny then miny = pt.y end
        end
        local width = maxx - minx
        local height = maxy - miny
        local avg = (width+height)/2
        width = avg
        height = avg

        -- make the coin GUI
        local coinGui = Create'ImageLabel'{
        Name = 'Coin';
        Image = 'http://www.roblox.com/asset/?id=121148238';
        BackgroundTransparency = 1;
        ZIndex = 1;
        }
        if
        at ~= Vector3.new(0,0,0) and
        pos.x >= 0 and pos.y >= 0 and
        pos.x <= ScreenGui.AbsoluteSize.x and pos.y <= ScreenGui.AbsoluteSize.y then
            -- onscreen
            coinGui.Position = UDim2.new(0, pos.x-width/2, 0, pos.y-height/2)
            coinGui.Size = UDim2.new(0, width, 0, height)
            else
            -- offscreen, just show it comming from the center of the screen
            coinGui.Position = UDim2.new(0.5, -20, 0.5, -20)
            coinGui.Size = UDim2.new(0, 40, 0, 40)
        end
        coinGui.Parent = ScreenGui

        -- tween the coin to the side display, and show the shownamount
        --Delay(0.5, ShowAmount)
        local function endTweenFunc()
            coinGui:Destroy()
            MyCoinCount.Value = MyCoinCount.Value + 1
        end
        coinGui:TweenSizeAndPosition(UDim2.new(0, 70, 0, 70),
        UDim2.new(0, 9, 0.4, 50),
        'In', 'Quad', 0.9, true,
        endTweenFunc)
    end
end)

Not sure, but I guess its not inside the gui. There is a frame around the 3D character, that is actually a normal model with a part behind as background, they just changed the camera position. If you notice, there is no other 3D space being shown in that screen, what means it's not a gui.

I'll tell you how they did it.

It's not possible to put a 3D object in a GUI because ROBLOX can only render one 3D space at once.

They placed a decal background as the "GUIs" and placed the bricks in front of the decal. They then locked your camera to be looking at it to simulate having a 3D GUI.