Is there a way i can do a function for Gui fading?? Need help with a gui?

Roblox has a service called TweenService which can be used for what you are asking. In fact, it can be used to tween almost any visual property such as transparency, position, and color. In order to use tween service, you first need to instantiate a TweenInfo object. Then you can pass this info into the Create() function in TweenService to get a TweenObject.

local gui = screenGui.Frame --example

--Create tween info
local easeStyle = Enum.EasingStyle.Quint
local easeDirection = Enum.EasingDirection.In
local info = TweenInfo.new(3 , easeStyle , easeDirection)

--Create goal for properties
local goal = {
    Transparency = 1
    BackgroundColor3 = Color3.new(1 , 1 , 1)
}

--Create tween object
local TweenService = game:GetService("TweenService")
local tween = TweenService:Create(gui , info , goal)

--Play the tween
tween:Play()

I actually developed this just the other day!

This needs to be in a LocalScript

local DEFAULT_DURATION = 2 -- seconds
local RunService = game:GetService("RunService")
local fading

local function lerp(a, b, t)
    return a * (1-t) + (b*t)
end

local function fade(gui, startTransparency, endTransparency, duration)
    fading = true
    duration = duration or DEFAULT_DURATION
    local elapsed = 0
    while elapsed < duration do
        elapsed = elapsed + (RunService.Heartbeat:Wait())
        gui.BackgroundTransparency = lerp(startTransparency, endTransparency, elapsed/duration)
    end
    fading = false
end

local function fadeIn(gui, duration)
    while fading do wait() end
    gui.Visible = true
    fade(gui, 1, 0, duration)
end

local function fadeOut(gui, duration)
    while fading do wait() end
    fade(gui, 0, 1, duration)
    gui.Visible = false
end

Example usage, appended at the bottom of the above script:

-- this example fades MyGui in and out twice
local myGui = game.Players.LocalPLayer.PlayerGui.ScreenGui.MyGui
-- first using the default fade time of 2 seconds
fadeIn(myGui)
fadeOut(myGui)
-- then using specific fade times
fadeIn(myGui, 5)
fadeOut(myGui, 0.5)

Bonus:

-- this example makes fading accessable via BindableFunctions
local myGui = game.Players.LocalPLayer.PlayerGui.ScreenGui.MyGui
local fadeInBindable = myGui.FadeInBindableFunction
local fadeOutBindable = myGui.FadeOutBindableFunction
fadeInBindable.OnInvoke = fadeIn
fadeOutBindable.OnInvoke = fadeOut