How would I go about controllable GUIs?

You can place this inside s local script inside the Gui Object you wish to move:

local UserInputService = game:GetService("UserInputService")

local gui = script.Parent

local dragging
local dragInput
local dragStart
local startPos

local function update(input)
    local delta = input.Position - dragStart
    gui.Position = UDim2.new(startPos.X.Scale, startPos.X.Offset + delta.X, startPos.Y.Scale, startPos.Y.Offset + delta.Y)
end

gui.InputBegan:Connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseButton1 or input.UserInputType == Enum.UserInputType.Touch then
        dragging = true
        dragStart = input.Position
        startPos = gui.Position

        input.Changed:Connect(function()
            if input.UserInputState == Enum.UserInputState.End then
                dragging = false
            end
        end)
    end
end)

gui.InputChanged:Connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseMovement or input.UserInputType == Enum.UserInputType.Touch then
        dragInput = input
    end
end)

UserInputService.InputChanged:Connect(function(input)
    if input == dragInput and dragging then
        update(input)
    end
end)

Too much to explain to be honest.

Hope this helps. :)

just listen for input events.. like maybe if a key is pressed or a finger is slid on the screen, you move the GUI in a particular direction, and things like that..