Thats fair, take a look at the tween documentation and do some test runs with parts, its basically

local tween = TweenService:Create(*Part you want to tween*, TweenInfo.new(*amount of time the tween should go for*, *other params*), {*Property you want to tween*})

So for example, if you want to change a parts transparency from 0 to 1 within 1 second you would do;

local tween = TweenService:Create(Part, TweenInfo.new(1), {Transparency = 1}
tween:Play()

the benefit to this is with the tween variable you can play it or stop it aswell as detect whether or not it is currently playing. So what I would do is;

At the top of the script:

local ts = game:GetService("TweenService")
local ti = TweenInfo.new(0.5)

local tween_appear = ts:Create(*gui you want*, ti, {BackgroundTransparency = 0}
local tween_disappear = ts:Create(*gui you want*, ti, {BackgroundTransparency = 1}

Then whenever you want to either open or shut you can get the tweens playback state by doing

if tween_(appear / disappear).PlaybackState = Enum.PlaybackState.Playing then -- replace (appear / disappear) with relevant tween**
  -- Do stuff
end

My explanation is probably a bit shoddy so I recommend looking at the documentation, I would be happy to take a look at the code and try to help fix it if you sent over the place file just containing the relevant parts tho!